erase Catalina volume?

MBP 13 2020 Intel purchased in Sep 2020 with Catalina

Jan 1 I installed Big Sur in a test volume. It's working great compared to the 3 months I tried to use Catalina - no more Thunderbolt 3 unplug crashes, work VPN is all set up and supported now, iCloud Photos downloaded cleanly in the background without the photoanalysisd hogging my CPU for over a month (really, it's still going if I boot in to Catalina).

I want to keep the Big Sur Test volume - I don't want any of Catalina's problems to linger and I don't want to have to reset up everything again when it's working well.

How do I erase the Catalina volume and data? All the instructions I find are to delete the test volume and upgrade Catalina to Big Sur.