In peertube how can I choose an instance and how can I have cross platform interactions??

idk how to have cross platform interactions.

Also, every peertube instance feeks very specific for my needs, am I supposed to just post the content on different instances? And even then what instance could I log in to? For example where would a viewer log in? I am not planning to only view content but still! Besides, does the instsnce I log in to could affect the communication speed with others? (I know they can like block eachother.) Anyways thanks a lot!