What is this extra partition that has appeared suddenly in macOS and how to delete it?
This is quite normal under BigSur and it is not a partition, but a snapshot of your drive. E.g. mine looks quite similar:

There is nothing you need to worry about and you should not try to delete it. This exact layout has been also discussed in Apple Forums, on MacRumors and on reddit.
To quote from reddit (user &quot;Advanced_Path&quot;):

Big Sur changes the way how updates are done. Now that the System volume is basically immutable, the way an update is done is with APFS snapshots. The system takes a snapshot of the base volume, updates the necessary file and from that moment forward, it boots from that snapshot.

Furher, I can recommend this article.