Steam and Unity Questions

1)* Is there a way I can start a standalone build and use the Steam Overlay?

If you launch the game from a shortcut in your steam library which points to the standalone build in your output folder then the overlay will be injected as the game starts.
I think if you use "Build and Run" in Unity that might work too, but only if you've already played in the editor in that session. (Because SteamAPI.Init() gets called, which injects the overlay, then build and run spawns your game as a child process and the overlay gets automatically injected into it.)

2)* Should I be able to start the game from the Steam Library?

Only if you've uploaded your game to steam and it's being correct downloaded into your steamapps/common folder. Your launch parameters in the Steamworks backend may be incorrect as well. (Don't forget to publish the changes!)

3)* If yes, should the steam_appid.txt file be next to the .exe or do I only need that when starting the .exe manually?

The steam_appid.txt is only required when testing locally, it's generally not recommended to ship it on steam to customers.

4)* Again, if yes, is it important that I put the build in the steamapps/common/my_game folder, or can the build reside in my usual UnityProject/Builds folder?

So, If you use SteamAPI.RestartAppIfNecessary, without having a steam_appid.txt then your game is literally doing a complete shutdown with Application.Quit() and then steam relaunches it once it quits. This relaunch is literally the exact same thing as double clicking the game in your steam library, so it kind of depends how you want to work. If you're testing with the overlay then having your builds output to your steamapps/common folder and just launching from your regular steam library icon may be best. Otherwise one of the other options above might be better.

REFERENCE: https://github.com/rlabrecque/Steamworks.NET/issues/142