Quick Note
I do not support JK Rowling and do not support other people paying for the game. I did not, however I legally do not endorse that either. I also am completely unsure how this actually works, this is purely speculative based on my own limited in-game research.
What this is not going to be.
This is not taking a peek at the code behind the system. I am looking at how the system appears to work from a technology perspective.
The Main Point
Hogwarts Legacy's portal system allows for engaging play if you do not test out the system properly, which is something they are semi-successful at preventing. You do not need any modifications to the game to test it out, and all my testing was done in the first instance of engaging with these portals.
How it seems to work.
Let's say there is a portal accessible from both sides. One side acts as the teleport-er, and the other allows you to walk through without being teleported.
The side that you can teleport through provides a full view of the destination, replacing the view that would be there otherwise. If you go through the portal, it doesn't seem to teleport you immediately, and I believe there is some rendering trickery until the player's camera enters the portal. This is because, if the portal is close to an edge, you can go behind the portal, standing on the side that doesn't teleport you, put your camera in front of the portal, and walk forward. It allows you to continue to walk, following the solids from the destination of the portal, until your camera travels through the portal. When your camera travels through, it reveals what you are actually standing on and, assuming you are standing on nothing, if you move you will fall.
The side that does not provide the teleportation allows you to walk through, nothing seems weird about that. I am unsure how they didn't get you to be triggered by the other side's portal, but that is not in my area of expertise.
In other places, it appears that the portal has its own version of the other side, simply teleporting you when you walk through. This is evidenced because of the fact that, in one instance, when you walk through the portal underground you can see the skybox in certain angles.
The issue with this system.
If the portal borders a ledge, your character is able to walk off that ledge assuming that the camera is looking into the portal. When it goes through the portal, it is clear to the player they are standing on air. If they do not move, the grounded check does not seem to run, which means they can stand in the air indefinitely. This provides no advantage to the player.
The portal instances where they render parts of the other side show an issue that seems to be something that they heavily want to avoid, showing the players parts of the map they would not see otherwise. The reason I know they want to hide these parts of the map is because of the EXTREME lengths they go to to prevent the player from 'escaping' the main playing area, even if it means breaking immersion. I can make another post on those lengths if requested.
These issues are not evident if the player simply uses the portal in the exact way that would be predicted by any party looking at the issue, but this is shown in places where your solving puzzles, and that causes people to do all kind of crazy things.
Why does any of this matter?
It doesn't. But, it can be useful if dealing with a portal situation in a game.