@irreticent I'm only on Mastodon/in the fediverse via browser so I can't comment on app options, sorry.
In my Home feed & Live feed, I see posts from all over and can reply, boost, and/or bookmark w/ only the one login. I can find & follow folks from other instances via basic search, with @ or a name if I know it.
Mastodon dot social has very active moderation. I'm sure posts & instances exist that I will never know about. Hope this helps!