by default game recording is disabled. by default it will use your GPU for recording (you probably want to leave this on). this works on windows and linux but on linux the only audio option you have is all system audio (does not include mic). you can either choose from background recording or record on demand.
with background recording steam will automatically record all games that have the overlay enabled and you can add markers with Ctrl+F12. background recording records up to 120 minutes at 12 Mbps, you can customize this and it will give you a total storage estimate.
with record on demand steam will not record a game with steam overlay until you press Ctrl+F11. you can stop recording again with Ctrl+F11.
valve intends for this to run on the steam deck as well and i've seen videos of it running on mini PC's so it's pretty light. game developers can also add timeline support, which includes changing the timeline color under certain conditions and automatically adding custom markers for in-game events.
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