Like @D.I. Johnson mentioned, it appears you did erase the system since there is only about 6GB of data on the "Data" volume.
There are three ways of erasing the data on the M-series Macs.
- Erase Contents & Settings
- Delete the "Volume Group"
- DFU Firmware Restore (requires access to another Mac currently running macOS 15.x Sequoia)
For the first two options, the following Apple article shows the proper instructions:
Use Disk Utility to erase a Mac with Apple silicon - Apple Support
Here is an Apple article for performing a DFU Firmware Restore which resets the security enclave chip, system firmware, and internal SSD (destroys all data on the internal SSD) and finally pushes a clean copy of macOS onto the internal SSD.
How to revive or restore Mac firmware - Apple Support
This is a factory clean setup. Anything you do during Setup/Migration is what is causing data to fill the drive. If you sign in with an AppleID/iCloud, then macOS enables all iCloud options so it will immediately start downloading stuff from iCloud. Personally I believe this is a terrible default option without asking the user what iCloud options to enable on the Mac.
FYI, There are actually three hidden APFS Containers on an M-series Mac's internal SSD with only one that macOS makes visible to the user ("Macintosh HD" and "Data" volumes). The physical nternal SSD is typically assigned "disk0", and each of those hidden APFS Containers are assigned device identifiers as well "disk1", "disk2", and "disk3" with the latter for the APFS Container holding the "Macintosh HD" & "Data" volumes.
When booting into Recovery Mode, then any volumes needed by the macOS installer are assigned device identifiers as well ("disk4", and "disk5") and perhaps more. Connected external drives can sometimes affect the numbering of these items as well. Just ignore any of the "Disk Images" while booted into Recovery Mode since they are just virtual volumes associated with Recovery Mode and the macOS installer.
