Use PostmarketOS Community pre-built image on Samsung devices

use Community pre-built image on Samsung devices
works running from an SD card

as explained in Installation - Flashing - From SD Card Our device can boot from an SD card, USB stick or other external storage.

preparation in a way to easily switch between Android_OS and PostmarketOS
from your actual Android_OS install file (AP_Stock_Samsung_Firmware.tar.md5 or Custom_Android_Based_OS.zip), extract the boot.img and copy it to your TWRP folder in internal storage (/sdcard/TWRP)

Download the pmOS pre-built image
from the download page

insert a MicroSDcard into your Computer
and find its name by running

$ lsblk

make sure it is the right one as you will overwrite everything on it. Use a path without partition number at the end,

such as /dev/mmcblk0, or /dev/sdd

decompress the downloaded file and write it to your MicroSDcard :
$ xz --decompress --stdout /path-to-pre-built-image-for-your-device.img.xz | sudo dd of=/dev/name-of-your-SDcard bs=4M status=progress

Wait until the command completes and eject the SD card.

insert the MicroSDcard into your device, and boot it into TWRP
using the mount feature, unselect all /partitions except "MicroSDcard",

using the install feature, switch to "install image" rather than "install ZIP"

navigate to the "/external_sd/boot.img", and flash it to the /boot partition

reboot system and enjoy
The rootfs image will boot right into postmarketOS. You are done with installing postmarketOS, congratulations! default login is user and password is 147147

to restore your Android_OS
boot the device into TWRP mode,

and simply using the install feature, switch to "install image" rather than "install ZIP"

navigate to the previously stored "/sdcard/TWRP/boot.img", and flash it to the /boot partition

to restore your PostmarketOS
boot the device into TWRP mode,

and simply using the install feature, switch to "install image" rather than "install ZIP"

navigate to the "/external_sd/boot.img", and flash it to the /boot partition