What are APFS system snapshots?

uncldrw wrote:

Here you can see that I got 2 Volumes.

I guess that "com.apple..os.update..." is used for the system snapshots

and "Data" is used for all other stuff like, documents, programs etc.

Yes, that is correct on both counts. With APFS, the macOS operating system resides in a volume group. By default, that group consists of: Macintosh HD & Macintosh HD - Data. The former is a sealed volume and contains the critical macOS system files. The latter also contains some system files but is mostly user apps and data. Also with APFS, a Container equals what used to be called a Partition. There must be at least one Container on a drive. In turn, a Container can have multiple volumes. Those volumes can "grow," as required, to fill the entire Container. APFS will manage those snapshots for you.

About Time Machine local snapshots - Apple Support

A "system snapshot" is used by APFS for a number of reasons and should not be deleted. For example, any time you update macOS, that process starts by first making a copy of the APFS system volume (Macintosh HD) to a snapshot. It then updates the snapshot, and when completed, signs & seals it to become the "new" Macintosh HD.

You may find the following article an interesting read, specifically the section covering macOS Big Sur:

• Boot disk layout on Intel and M1 Macs: High Sierra to Big Sur - The Eclectic Light Company

You do NOT have a 512GB SSD.

Where did you notice this Systemsnapshot?

Big Sur OS is mounted via a sealed snapshot, but when you select it in Disk Utility, the storage bar will show it combined with all of your data, not just the OS.

Did you migrate data to the Mac? Is most of that taken up by "Other Volumes?"

The question is, will my mac use the remaining volume if "Data" gets full?

That large black bar on the right side is what is free to be used.

The blue and gray sections are what has been used.

However, I have no idea if you really have a 512GB SSD. It looks like you have a 256GB SSD.

Click on the View button (Darstellung) and choose "Show all devices."

Look for an additional Container on that drive (or post another screen shot).