Factory reset via the hardware button on the back of the device sets the boot argument to the @factory BTRFS subvolume on the eMMC’s mmcblk0p1 partition.
Medkit reset from a USB medium resets the partition table of the eMMC, reformats the eMMC’s partition and sets the boot argument to the @ BTRFS subvolume on the eMMC’s mmcblk0p1 partition.
With the change of the boot argument it will mount the eMMC instead of the SSD.
It been hinted here
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Another option is a secondary partition on the SSD for TOS5.x, leaving the TOS3.x partition untouched. To switch the partitions being mounted during boot it would require to change the boot arguments.