If you’re looking to make your own portal-based game, here’s a list of stuff that I found to be really helpful:

Incredibly helpful talk by Portal creator
Handling oblique near plane clipping
Brackeys’ video on doing portals
An open-source portal project, but with visual glitches
And the blog post for that project
Someone else doing a similar thing for VR
A good video showing how to prevent clipping of the portal plane when the camera gets close

Most of that info came from the incredibly helpful responses to my dumb question. I personally wouldn’t recommend doing this with stencil buffers because it can break the way Unity renders some effects, but if you still want to do it that way, here’s a couple of relevant posts:
https://unseendiplomacy2.com/2019/04/portal-rendering-part-2-rendering-portals-in-ud2/
https://www.ronja-tutorials.com/2018/08/18/stencil-buffers.html

This is the development log of my portal-based experimental game. I’m developing it in Unity as a singleplayer puzzle game. I’m a big fan of Portal, and I’ve always wanted to create a portal-style game. I also want to create something a bit unique, so it’s not a direct clone of Portal.

The “experiment” part is that I have no idea if I’m capable of completing a game. The goal is to give myself the greatest possible chance for success, so I’m making a game that is singleplayer, includes minimal dialogue, relies heavily on very technical programming, requires minimal artwork and can still tell a deep, interesting and (hopefully) exciting story. In other words, I’m attempting to play to my strengths.

I’ll post whenever I feel I have something interesting to share, and include as much technical detail as possible. I want this to be a permanent record of the development of my game. Well… as permanent as anything on the internet can be.

Here’s a list of the few Reddit posts I’ve already made about the game:
Most recent demo gif (same as above)
Original demo gif of seamless portals
Explanation video for how the simple seamless portals worked (not very good)

And no, the game has no real name yet.