What OS is on the Recovery Drive? (and related questions)

After a failed Boot Camp install, my 2017 iMac can't recover the space Boot Camp partitioned. The Boot Camp Assistant, when I tried to remove it, said that it failed, and when I opened Disk Utility in Recovery mode it doesn't see the partition, but the space hasn't be reallocated to my main hard drive. It shows my Fusion Drive as having 3.12 TB and the only disk visible within it is "Container disk2" which has my "Macintosh HD" which both say there's only 2.74 TB allocated.

So I'm thinking I need to reformat the Fusion Drive and reinstall the OS and restore from a Time Machine backup. My question is, the iMac came with Sierra or High Sierra (I can't remember which) and I'm currently running Mojave. I'd like to restore it with Mojave and not Catalina (because of software I run that doesn't work on Catalina yet). Is this going to be a problem? Are there steps I have to take, or does my Recovery Drive have still have the OS it came with or Mojave since it's the OS I'm currently running?

I've looked online for an answer, but it's not clear from what I've read what version I can expect to be able to install from the Recovery Drive. I can work with a Sierra or High Sierra and upgrade from there, but I'm not sure of what to do if Catalina is the version it wants to install.

Thanks for any help!