Comment by pmontra
Samsung DEX had a Linux desktop package in 2018. It was a lxd container based on Ubuntu 16.04. They developed it in collaboration with Canonical. Unfortunately they deprecated it shortly after, maybe already in 2018. The next Android update would remove it.
It worked but Android killed it mercilessly if it used too much memory or the rest of the system needed it.
Some current Android devices that have USB-C 3.1+ and support dp-alt-mode (USB-C to HDMI) will detect when an external display is connected and provide a full extended desktop. [0]
You can connect mouse, keyboard, and display to the Android device through an unpowered USB-C hub that offers the respective ports. Battery life depends on the make/model of Android device.
I have a Motorola phone and the experience is very nice.
[0] _ https://uperfect.com/blogs/wikimonitor/list-of-smartphones-w...