How do you make demo versions of your game?

With submissions to Steam Next Fest coming up, I'm almost at a point where I can upload a demo build to Steam, but I still need to separate what is considered the "demo" and the "full game".

Do you typically fork your project and work in a new project for the demo? Do you just add some kind of gating to the main project that limits what is accessible in the demo? (Does steam support this method, i.e. using GodotSteam can I determine the version of the game and adjust the game based on the result?)

I have concerns for both methods, the first being if I fork the project, I have to do double the work to make changes that can affect both versions of the game. The second, is that shipping the full game files with the demo would result in larger download size for the user, and though I know it shouldn't matter much, but the demo could then be decompiled and adjusted to provide access to the full game. Ultimately, users who are going to pirate/alter a game are going to do it whichever way is easiest, so that may not be a problem I should really concern myself with.

What are your thoughts, how do you do it?