The first partition on the emmc should be have an id of ef and start at sector 2048.
Best way to recover to the originl OS is to download an image from DL, unzip, copy it to a bootable usb stick and run this.
dd if=/where/you/put/the/image.img of=/dev/mmcblk0 bs=1024k oflag=dsync status=progress
Images available at:
Readme for the images:
When I have installed a new version of Linux on the emmc I leave the partition table as is and install to the Linux partition, just choosing to reformat and mount at / (root)
The following is the output of running fdisk -l on one of the unzipped images. I think on first boot the root partition is resized to fill the disk. (formatting is a bit out) but you should get the idea.
atomicpi_debian_buster_lxde_standalone.img: 7 GiB, 7462318080 bytes, 14574840 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xfd9972cb
Device Boot Start End Sectors Size Id Type
atomicpi_debian_buster_lxde_standalone.img1 264192 14574839 14310648 6.8G 83 Linux
atomicpi_debian_buster_lxde_standalone.img2 2048 264191 262144 128M ef EFI (FAT-12/16/32)
Partition table entries are not in disk order.