You can add them as shortcuts. UWP need a tool to set them up in steam, but you can do them too.
They’ll still need to open the other launchers to run, but it’s about as close to seamless as you’ll get. You can make Windows log in automatically, not lock when it sleeps, and launch to steam big picture mode. It might take a search or two to set up, but after it’s (minus windows being windows) relatively out of your way.
You can do the same with Linux, but you lose anticheat games and setting up other launchers is more work.
You don’t need any extra tools. A basic function of Steam is “Add non-Steam Game” it’s the little + symbol at the bottom left of the window. Find the .exe of the game you want to add, click, and it’s in your steam library now. You’ll want to add custom artwork, but it works. If the game launches in a separate launcher, that launcher will launch from steam of course.
