sorry but my english is not high level and I don't want to make something wrong...
can someone write me what I have to do with simple words?
About Signed System Volume
Excerpt from an Apple article:
In macOS Catalina, Apple introduced the read-only system volume, which is a dedicated, isolated volume for system content. Now, macOS Big Sur adds strong cryptographic protections to store system content on a signed system volume (SSV). SSV features a kernel mechanism that verifies the integrity of the system content at runtime, and rejects any data — code and non-code — that doesn’t have a valid cryptographic signature from Apple.
SSV helps prevent tampering with any Apple software that is part of the operating system. Additionally, it makes macOS Software Update more reliable and much safer. SSV utilizes APFS snapshots, so if an update cannot be performed, the old system version can be restored without reinstallation.
More notably:
Signed system volume is transparent for most developers. Kernel developers who must interact with the root volume can do so via a new System Integrity Protection mode available in the csrutil command line tool. When you enable this mode, it allows the tagging of custom snapshots using apfs_systemsnapshot, which you can then boot into.
Bypassing SSV
So the way macOS Big Sur is supposed to boot is that it never boots off the live volume, but a snapshot of it (its not happening on all systems though). Naturally, it is impossible to mount a "snapshot" as writable. So what can be done is as follows:
In recovery Terminal:
csrutil authenticated-root disable
As of Beta 1, FileVault needs to be disabled to turn off ART.
Back in normal boot, mount the live volume as writable. This is possible as on Catalina:
diskutil mount disk1s5 # Use your live volume's disk identifier.
sudo mount -uw /Volumes/Macintosh\ HD\ 1 # Use your live volume's name.
Make modifications to the live volume.
Generate and tag new APFS System Snapshot as follows:
# Create snapshot.
sudo /System/Library/Filesystem/apfs.fs/Contents/Resources/apfs_systemsnapshot -s "SnapshotName" -v /Volumes/Macintosh\ HD\ 1
# Tag snapshot for next boot.
sudo /System/Library/Filesystem/apfs.fs/Contents/Resources/apfs_systemsnapshot -r "SnapshotName" -v /Volumes/Macintosh\ HD\ 1
Reboot and voila!
