GraphQL sample code via plain HTTP posts missing

Supabase advertises with "Query your tables, including filtering, pagination, and deeply nested relationships (like GraphQL)". Is this only possible via supabase-js (and other clients), or also via plain http payloads?

In general it was already hard to find info on how to query PostgREST as nowhever was mentioned you need to add "/rest/v1/" between your hostname and the manual.