Filter API results with relational queries

I’m trying to figure out how to do relational queries or “search on references” based on these two doc pages:

I’m having trouble following this requirement as part of the request:

Second is which you use to to filter on fields of entries from content type 2PqfXUJwE8qSYKuM0U6w8M.


Where does one find the value?

What does the content model look like?

Same with the below example:
   &[all]=Universal Studios

Specifically value?

Thanks for any help!