Mojave to Monterey, on MacBook 2015

if it were me, I would upgrade one macOS at a time all the way up to Monterey. while it is possible to jump past macOSes and upgrade directly to Monterey, some users have encountered issues after doing so.

you can find all of the downloads here: How to download and install macOS - Apple Support

at 10.15 Catalina, 32-bit apps ago unsupported, and the separate System Volume splits off and becomes un-writeable.

at MacOS 11 Big Sur, the separate locked System Volume is crypto-signed for tamper detection, and the default Time Machine backup format going forward becomes APFS.

I agree with jeffreythefrog that you may want to do this in multiple stages, at least through those versions I mentioned.