QCC is a Quake 1 mod — it does not use Quake Champions. Buy Quake on Steam or GOG, then locate id1/pak0.pak in the install folder.
Download the FTEQW client from fte.triptohell.info — runs at 4K with full PBR and connects natively. Competitive alternative: ezQuake. Place the QCC mod in a qcc/ folder beside id1/; the server auto-downloads missing mod content on connect.
Launch QC with QCC enabled and connect via the server browser:
qcc.sonofasam.com:26002
qcc.sonofasam.com:26002
No password. Champions each have one active ability — otherwise classic Quake deathmatch rules.