clean install: you make a backup, nuke the computer, install a fresh upgraded copy of the distro you want from a live usb, copy your data again to the computer.
upgrade: you wait ‘till the distro’ developers release an upgrade you can directly install from your soon to be old distro, you use a command like sudo do-release-upgrade
and why do you upgrade like that?
I feel like that may be true nowadays, but I remember back when I used to use ubuntu that the upgrade from 16.04 to 18.04 was pretty bad. Fedora has always worked great for me, but these days I only use rolling release distros in which case there aren’t any major version updates in the first place, so the problem largely doesn’t exist in the same context.
Canonical makes ubuntu makes upgrades break on purpose so they can sell you ubuntu pro that has the fix in it. For example the upgrade you mention broke grub but only the paid support release ring/branch has a fix