I mean...I'm not American so have no view on your parties but otherwise, yes you got the idea. That's what a competition authority is meant to do. Of the ones you mention there's investigations into Google search, Microsoft was already convicted for Windows but wriggled out of it, Apple and Google are both being investigated for practices on iPhone/Android respectively, Apple Music/Spotify both complained about dominant abuse and I think there was some investigation (I know there were complaints, not keeping up to date with outcomes), Coke and Pepsi are enormous if you consider their sub-brands too and I completely agree with looking into the food market (throw in Unilever as well), Amazon's abuse of dominance has led to many fines, Walmart - again not American but what I read about them going in at a loss, waiting for things to close/go bankrupt and then raising prices all the while claiming subsidies for their workers - yep, agree they should be investigated. Steam absolutely should do - it has far too much dominance. Meta and Tiktok both were investigated...
Yes, you're pretty much spot on with what a competition authority should be doing.