How to Dual Boot two versions of Mac OS

In descriptions of Dual Booting one is given a picture of a choice of destination between two Volumes, one, the original Mac HD and the other, a constructed volume defined by addition by Disk Utility. This is conceptually straight forward. However what is never mentioned is how one gets access to a clean operating system version that is not encumbered with a predestination to be an upgrade to the resident os already on the Mac HD original volume. I am running an i7 Mac Mini with Mojave 10.14.6 and I want to dual boot up in the second volume a clean version of Catalina or possibly Big Sur. Finding one or the other of those installer packages not predestined to be an upgrade to Mojave has eluded me. It appears impossible but I am hoping someone knows differently.

Thanks for your attention

RAZ