Filter API results with relational queries

APIs Content Delivery API
1

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 fields.brand.sys.contentType.sys.id=sFzTZbSuM8coEwygeUYes which you use to to filter on fields of entries from content type 2PqfXUJwE8qSYKuM0U6w8M.

content_type=2PqfXUJwE8qSYKuM0U6w8M
&fields.brand.sys.contentType.sys.id=sFzTZbSuM8coEwygeUYes
&fields.brand.fields.companyName[match]=Lemnos

Where does one find the fields.brand.sys.contentType.sys.id value?

What does the content model look like?

Same with the below example:

https://cdn.contentful.com/spaces/my-space-id/entries/
   ?content_type=movies
   &fields.genre.sys.contentType.sys.id=genre
   &fields.genre.fields.label[all]=Thriller
   &fields.productionhouse.sys.contentType.sys.id=productionhouse
   &fields.productionhouse.fields.name[all]=Universal Studios

Specifically fields.productionhouse.sys.contentType.sys.id=productionhouse value?

Thanks for any help!