Based on https://forum.xda-developers.com/t/dumping-stock-firmware-to-use-for-lineage-custom-build.4034621 by dcrin/derfelot
Find AIK-Linux in the dev sections on xda containing unsin and unpackimg.
Three .sin files from stock are needed: vendor, kernel, system.
Extract them with unsin.
You will end up with one .img file and two .ext4 files.
kernel_X-FLASH-ALL-C93B.imgsystem_X-FLASH-ALL-C93B.ext4- `vendor_X-FLASH-ALL-C93B.ext4
Unpack the kernel with: ./unpackimg.sh kernel_X-FLASH-ALL-C93B.img
Own your files: sudo chown -R $USER:$USER ramdisk/
Note that the ramdisk folder will be in the same folder as unpackimg.sh independent from where you called that script from!
Mount the ext4 images:
sudo mkdir /mnt/system && sudo mount -o loop system_X-FLASH-ALL-C93B.ext4 /mnt/systemsudo mkdir /mnt/vendor && sudo mount -o loop vendor-FLASH-ALL-C93B.ext4 /mnt/vendor
The kernel copy you did previously gave you an empty system and vendor folder.
Copy all contents of the mounts into those empty folders.
Finally rename ramdisk to something more appropriate and use it with ./extract-files.sh /location of extracted contents.