Vulkan and OpenGL are not engines but a graphics API:

https://www.vulkan.org/learn

https://www.opengl.org/

These are engines:

https://en.wikipedia.org/wiki/List_of_game_engines

I definitely read your comment not correctly 😂

But yea, soon, most devices can run Linux apps just like that, per default

😆macOS (and it’s various variants of course) is then the only OS not coming with a container/VM built in it running linux 🤔

Most mac gamers do not game via steam, I’d say, you have to look at the apple app store as well.

Is it harder to port from Linux to windows than other way around or does that not really matter?

Or are there engines to use, that are already globally supported by all 3 big OS?

If you count apple, you can count android already now. Android is more Linux than apple iirc.

Android is linux based and macOS is BSD based, again, iirc.