You're at the ~700 level. There'll be some players who know a bit of theory, but a lot of the time, people are just making moves and not understanding why.
I think it's best to focus on an opening you find the most fun in! That way you stay interested in the game. Having said that, it's hard to sometimes play King's Indian Defense as it can be quite a passive opening. If you know an opponent has played a move which isn't good, then it's up to you, through training your tactical vision, to find out why.
You seem to know about chess principles at least, when you suggested developing moves. If you can't see any tactics, for you or your opponent (when your opponent plays a weird move), then just continue your plan and see how you go! It's only experience that will get you there.
Most videos I see online say something along these lines: Once white plays this move, black will respond with either move a or move b, and anything else is a bad move or losing for your opponent. More likely than not at the beginner level I never see the book moves and usually lose to a move that is considered bad for my opponent but one that I don’t have anything prepared for.
Is there a particular opening that I should focus on as a beginner, or is it best to just respond move by move to what my opponent does? Any advice is appreciated.
Recently I have been trying the London system as white, and the Scandinavian or Kings Indian as black.