While setting up Ubuntu on the Raspberry Pi is easy, there isn't an official Ubuntu desktop image for the Pi. Actually, the only official image for the Pi is the IoT focused Ubuntu Core. That's probably not the version of Ubuntu that you're thinking about for your Raspberry Pi projects, though.
Instead, there are a couple of options to get a more traditional Ubuntu experience on your Raspberry Pi.
This is a companion discussion topic for the original entry at https://linuxconfig.org/install-ubuntu-16-04-mate-or-ubuntu-18-04-on-raspberry-pi