READ THIS CAREFULLY, DON'T JUST DOWNLOAD IT BECAUSE IT WON'T WORK!
Download BootCaT from here and install it:
https://bootcat.dipintra.it/download/BootCaT-1.57_Intel.dmg
If you want the Apple Silicon version download this (if you're not sure what “Apple Silicon” means, then use the link above):
https://bootcat.dipintra.it/download/BootCaT-1.57_Apple_Silicon.dmg
NB: you'll need to do this only once, after the first time, BootCaT will open normally.
Since version 10.15.X “Catalina”, MacOS prevents non-notarized applications downloaded from the Internet from running.
Since BootCaT isn't notarized, the workaround presented here is needed to and run the App.