Just thought I’d share how I installed Raspbian on the onboard eMMC. I got the x86 Raspbian image from It’sFoss https://itsfoss.com/raspberry-pi-os-desktop/, and I used the Balena Etcher app to flash the USB drive. It was pretty simple, and it works a treat now. Interestingly, although ItsFoss says the image is 32 bit, it installed with 64 bit architecture on my Atomic Pi. The cursor still has that annoying stick at the left side of the screen for some reason, but I much prefer Raspbian over the stock Lubuntu. A lot less buggy.
ETA Prime has a great OS install tutorial: https://www.youtube.com/watch?v=zM7HwdaUv0o