I used to know how to do this, but forgot (been a long time since I've used Lineage).

Don't know if "factory reset" is even the correct term here, but I'd like to wipe everything on the phone, files, apps that don't come bundled with the OS, everything except Lineage itself. I think you have to go into TWRP (???), and then . . . ? Anyway, would be much obliged for any help with this.