The primal client doesn't connect direct to relays, it connects to a cache that's populated with unknown list of relays on the backend.
Discussion
Not unknown. Everything's public.
https://github.com/PrimalHQ/primal-caching-service/blob/main/relays.txt
this list of relays is the most interesting lately:
https://github.com/PrimalHQ/primal-caching-service/blob/main/relays-mined-from-contact-lists.txt
periodically i run a script to mine relays from all collected contact list events.
https://github.com/PrimalHQ/primal-server/blob/main/collect-relays-from-contact-lists.jl