Part of this is explained because ports are both artificial and natural. Artificial portals require a 'key' (a THz frequency oscillator at JUST the right value - to 18 decimal places!). 'Natural' and artificial portals can be triggered by individuals having some innate extrasensory characteristic (maybe some ESP variant of the 'THz' key - they can 'send' at just the right frequency to trigger a portal.
Zinn is able to pass thru a portal because she is a 'natural' and can speak to the Sentinels.
Other 'non-ESP' or not carrying a 'key' can walk right thru the portal and not trigger anything - nor is the portal detectable by any human known technology in the Nexus timeline without one of the other capabilities.
The creation of the natural portals is a mechanism unknown. Maybe an ancient race. Maybe cracks in the creation of the universe (doubtful because they aren't random - they go to planets). The artificial portals are 'Mark 2.0' or some other races creation that happens (maybe?) to overlap on the Nexus. Maybe the '2010 Probe Exchange' met the 'Stargate commons' met the 'Contact exchange' met the 'B5 Hyperspace'.
Portals deactivate certain levels of technology going thru them. A hammer will go thru. A wholly mechanical 9mm goes thru. A blaster doesn't work on the other side. Data storage devices that work on charge coupling, electron spin, magnetic domains, stored electron voltages, delay lines, or capacitance all fail. Devices that work with reflective 'pits' are good down to 500 nm (about 15% smaller hole size than current generation Blue-Ray DVD's). The faint deep blue aura around someone coming thru a portal may be due to the fact that electronic device semiconductors show many many 50 nm holes when examined closely after going thru a portal. A guess, never proven using any instruments since they fail to show what people 'perceive', is that the 350nm color is due to some radiation/static effect in a portal that makes junk out of so much electronic gadgetry. Thus a gadget using Super Blue Ray type technology, with huge amounts of additional coding, may carry about 1TB (user data) thru a portal. Denser than that simply fails due to some unexplained value changes inside the portal. For like reasons, cybernetic intelligences, AI's, and Hithiker's Guides to the Galaxy simply don't transport thru a portal. Maybe gods made them that way. The problem is, of course, that you may end in a place with nothing to 'read' your user data storage device (that is roughly the size and shape of a very thick DVD).
Portals can end up in space (floating unattached to a planet), but most of the time they materialize in some fixed relationship to their planet. Unfortunately, the planet may move/tectonically migrate such that after some eons of time the portal might be inside rocks, above or below the ocean, next to a building, or be blocked by debris.
Powered armour, protective suits, and special gear don't really protect you in a portal. There is no 'time interval' subjectively going into a portal, nor is there any 'partway thru' experience/possibility. Once a 'qualified object' breaks the plane of a portal it is instantaneously (subjective to the person being transported) pushed out the other side of the destination portal. A "shwoup' sound may be heard as the air rushes into where the person used to be. The actual time (objective to the starting end) measured by going thru and coming back (an immediate turn) may be up to 1500 minutes each way. People going thru a portal, even though not experiencing any time, need to 'cover up' or they come out with faint skin hemorrhages like they had been exposed for some moments to a very high vacuum. These behave on people like slight burns (or in the case of eyes, loss of vision for a short period due to blurring and tearing in the eyes).
People didn't create the portals - they stumbled upon them and still don't really know how to work things. If the Time Patrol agents from the advanced timeline really know - they are talking about it.
There is a mass limit going thru a portal. Maybe the inventors/discovers were health fanatics, or very small, or didn't need to carry much, but a mass limit of about 250 kg (550 lbs) limits 'shipping' thru a portal. Anything bigger than that just goes along - ignored by the portal. Anything small (like the atmosphere or snow or blowing dust/germs) doesn't go thru either (too small or something). So, a portal is 'germ proof' to what's on the other side. A portal is temperature, heat, light, vibration, chemically, and penetration ignorant of its two sides. Nobody knows why. If you don't have a key or the right brain, then you just pass right thru a portal interface plane like it wasn't there. So there's no visual sign or physical sign that a portal is there until the right key or right brain happen to break the plane of the portal. Only the feet of a sentient can go thru - so rolling wheelbarrows of stuff thru is out. You can also go thru in mid-air or falling. For unknown reasons most portals are aligned with their 'faces' 90 degrees perpendicular to the planetary face ('doorway alignment') But not every one. Most portals are parallel with gravity on their planet (no falling out at 30000 feet to fall) at or near a surface (or what was a surface probably). Portals keep their alignment with a planet (but the 'doorway' edges may go into any colinear directions - no curved portals).
Oh, and something does happen to 'biologicals' clinging to a person going thru the portal. Sentient beings hand-in-hand will go thru, but dumb animals (like a wolf hanging on a person) comes out dead on the other end. Bacteria on the skin surface sometimes die, and sometimes not (due to the length of objective time in a portal maybe?). Leeches, ticks, flies, mosquitos, and all the sorts of skin parasites die - but tapeworms might not. If you stumble out unplanned onto the Nexus the sentinels make sure you get 'cleaned out' so that you aren't a hazard to the health of the Nexus. Nobody living knows how this works - more of the Time Patrol's Timeline 'magic'. The portals can somehow differentiate between a domestic cat, a tiger, and a Sentinel. The portals don't let chimps, baboons, Neanderthals, or Cro-magnan thru. The portals can't send biological specimens back very easily thru the portal. The tissue samples from a microtome show up as degraded. The biological fluids come out in a soupy state of proteins and aminos without form. A baboon with a 'key' in its paws just bounces off a portal - not enough something, maybe brainpower. A child with a key might, or might not, go thru. An alzheimers patient won't go thru. A psychotic might or might not go thru.
Portals aren't physically large either - about two meters tall and one meter wide (about) with some kind of 'edge' that you can 'richochet off the edge into the portal'. On he other hand - if you collide with them 'edge on' there is no thickness - even a key holder or esper will talk straight thru without being affected. This leads to an interesting conundrum with portals - they can be 'right next to something' or even co-planar and no-one knows they are there. Some portals are smaller and rounder - you can crawl thru, but not run.
Portals are not 'time synchronous' as in you can leave a planet at high noon and end up at deep midnight on the other end. This also extends to changes in temperature, oxygen (up a mountain), pressure (down in an ocean), atmospheric content (destination planet is only 5% oxygen now). Time rolls on at each of the portal at the same rate (entropy is maintained), but the travel time of up to more than one day can be used as a 'person delay line' storage device for your consciousness.
Portals aren't distributed around the nexus in any pattern anyone can perceive. You might have one in the middle of hundreds of miles of no others, or in a tight geographic cluster of 20-30. Portals are thought to exist around the space of the nexus planet, but no one has ever bothered to try and check. Portals may be inside mountains, hidden in jungles, or under water on the nexus. Nobody knows. Portals can be closed by Sentinels and cannot be used (locked out somehow?) Nobody knows how they do it. Some portals are locked 'from the other side' and you can't go in them even with a key or esper.
Portals are 'full duplex' (you can go instantaneously thru from each side at a time). Portals may physically move, but never change destinations. They are linked to two, and only two, places in spaces. Although, some portals may become 'unstuck' and end up at a different point in the same destination space and time. With no pattern to the variance and no warning that this could happen. One theory is that portals in space became unstuck somehow