A few Steam games can be played without needing Steam. This requires finding the game's executable in your system and running it. A full list of games that can be played this way are here.
However, for the most part you seem to be able to play Steam games offline once they're installed.
This is not the case for all platforms. For example, with Epic Games, you can only play "your" games by first logging in with the app, even if they're already installed. The same is true for the Blizzard Battle.net launcher (it might be possible to play offline if you log in once every 30 days or so online, but it didn't work when I tried to launch them while offline even when I had already previously authenticated).
The EA App does let you play your games offline, it seems, once they're already installed (but not without lauching them through the EA app).