If you don't like autologin in Armbian Ubuntu desktop, or if yo want to have several desktops to choose, here is how I did this.
Tested on legacy image, but I belive that same setup should work on mainline too.
1. Install ubuntu desktop image on MMC (I did it using Etcher)
2. Install display manager. Because of simplicity, I choose LXDM.
sudo apt-get install lxdm
3. Set default session and disable nodm