I designed and built a handheld PC (Decktility)
github.comIt's built with a Raspberry Pi CM4 and a 5" touch screen. It has about 6 hours of battery life and batteries are replaceable. The keyboard isn't great, but it's functional.
Inspiration and motivation came from Yarh.io, uConsole and various handheld PCs from the past.
Some pictures: https://imgur.com/a/ug10swu
So they didn't 'design and build a handheld PC', they designed and built a case.
I guess it should be able to run something like https://mobian.org.
Interesting - never heard of it. I found another project(*) that runs Mobian on a Pi, so that should work. I intend to try out ParrotOS and/or Kali in the future. Perhaps some RTL-SDR tinkering too.
(*) https://liliputing.com/ourphone-is-a-diy-raspberry-pi-powere...