At Google? L5 is a reasonable terminal level there, as in you can have a successful career and never go beyond L5, and no one will bat an eye.

L6 is a senior position, and not just in terms of title.

L4 is now a reasonable terminal level as well, if you, for whatever reason don't want to, or can't push to L5.

Yes, before an on-site.

Your interviewers and and the type of questions asked depend on your potential level.

The expectations and responsibilities of an L4 and L6 differ so strongly that I'd be interested in a hypothetical candidate which could be both.