I'm just looking for some clarification, after realising that many of my essential apps were incompatible with Catalina (rookie mistake, I know), I created an installer thumb drive with Mojave on it, and reverted back to it. I understand that with Catalina came the separation between the former 'Macintosh HD' and 'Macintosh HD - Data', but what I was not expecting was for that to remain once I went back to Mojave, and now I am not even sure which volume I should be installing apps on..