How do I subscribe/view/post to other instances

I know I've been asking a lot of questions recently, I'm just trying to learn and gather info about this new place

I understand that the instances are like email. I have an "Gmail" account that can see and send messages to an outlook account, but I can't use the Gmail to sign in to the outlook website. My problem is that I cannot figure out how to view, subscribe, and post to other instances from mine. I have tried searching for them using the !(communityname)@(whateverinstance).(something) format but nothing seems to come up, and the only time it has it just brings me to that instance and I couldn't do anything since I didn't have an account on that instance. am I missing something?

For extra info I am mainly using the android app (from GitHub not the Play store), and my main account is on lemmy.ml

Thanks for any help, hopefully I can stop inandating you with questions soon enough and start to fully contribute to the fediverse!