I have used MDS to reinstall Catalina on a 2018 MBP, but this same process does not work to downgrade to Mojave (install Mojave dmg on a machine running Catalina). Is it supposed to work? Do I need to do something different to get it to work?
you need to boot from Mojave (or earlier) to install Mojave. This is usually done by booting to an older internet recovery partition (command-option-shift-R).