Upgrading & APFS Volume Advice Needed

My iMac is operating on Mojave 10.14.6 but I want to upgrade to Catalina or Big Sur. As I have some apps for work & games that I play that will not work under the new OS, I created a separate APFS volume according to these instructions: https://support.apple.com/en-us/HT208891. I see it sitting there in the finder and of course it’s empty - I named it Mojave WG. I’ll eventually get rid of it once apps are updated but for now, I need to have the ability to switch OS in order to use them.

But now I’m unsure on how to proceed with what I want to do. I want to install Mojave (my current operating system) on the newly created APFS volume ‘Mojave WG’. I then want to upgrade my computer to Catalina or Big Sur (not sure which I want yet). I planned on downloading Mojave and selecting the APFS volume ‘Mojave WG’ to install it on (if it offers me to select that). Then I want to upgrade to Catalina or Big Sur from the software updater on my computer. What I’m unsure of - is that the correct way to do it? Will upgrading wipe out Mojave on the APFS volume our does it ask you specifically where to install it? Or do I have to use MacOSRecovery as that page says? 

Thanks in advance for any advice anyone can give me.