A handheld Linux terminal using Raspberry pi Zero 2W as Core with 4“ 720×720 TFT display and the original BlackBerry Keyboard.

The Raspberry Pi Zero 2W only has 512MB RAM, which is a strong limitation.
This severely limits the usage of a GUI and it is recommended to use it mainly in the CLI. Furthermore, some distributions (mainly Kali) will not even update because they run out of RAM. To fix this issue, it is recommended to add 2GB of SWAP.

The Raspberry Pi Zero 2W connects with the PCB of the Hackberry through spring-pins and is held in place with screws. If the pins are misaligned or don't make a solid connection with the pads on the underside of the Raspberry Pi, the display might not work or show only two color channels.
To resolve this issue, clean the underside of the Raspberry Pi with cleaning alcohol (isopropyl alcohol/IPA) and tighten it down with the four screws until it sits somewhat loosely on the spring-pins. Wiggle the Raspberry Pi until the spring-pings have clicked into place. Then tighten it down.
If you can't make a solid connection after several attempts, tighten down the Raspberry Pi, turn the Hackberry on, and then release the screws a little. Start wiggling until the colors display correctly and tighten the Raspberry Pi down gently.

The batteries are connected in parallel without any current protection. According to Zitao, this isn't a problem because the batteries have enough internal resistance avoiding any high currents between them if they have different charge levels.
Some users report that one battery discharges quickly while the other doesn't discharge at all. However, this is a very uncommon issue.