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This is my computer setup.
(cdn.masto.host)
Linux as my daily driver via Steam Deck docked to my 65" tv at 1440p 120hz. The Steam Deck isnt only a handheld gaming console. It's a full Linux machine.
The "start button" is the kde plasma logo. So this would be Linux of some sort (makes sense given the community and what OP has said) and not windows
The question is just whether OP is using steamOS that comes on the deck (and uses KDE plasma for desktop mode) or if they have installed a different distro that fits the desktop use case a bit better.