I need to second this one! Tried some of the apps that came out in the past couple of weeks/days and Liftoff is the one I stuck with. The button at the top where you can switch to different instances with a single tap actually made me better understand the whole way Fediverse works. Now I switch between Beehaw, Lemmy world and some of the more obscure ones super simple (I am sure one could do this with other apps also but with Liftoff it just clicked for me).
My homescreen is still littered with a lot of the same stuff because a lot of instances have similar stuff on them but this is the first time I have something resembling control over the content I am reading in the Fediverse.
(still mourning the loss of RIF and it's simple and incredible easy to use interface. I truly hope RIF's dev makes an app for Lemmy!)
I am pretty sure that you can upvote most of the stuff on instances you don't have an account for. However some instances require you to apply for an account rather than just singing up (eg Beehaw was like that for me). An admin has to manually click a button for you to become a member. In those cases you can not just upvote or comment on one of those instances without an account. I think it's to prevent a flood of bots on some of the instances. Not entirely sure if I got it right, though!