It seems you may have re-installed Big Sur at some time but without deleting the Big Sur Data volume from the first installation. That's why you've got 2 of them.
You have 2 choices, as I did when this happened with Catalina on mine.
You can delete the Data volume that is not linked to your system volume and see if that has any impact on your running system.
If it doesn't then you're set.
If it does have an impact (things missing or not working) you'll need to delete all the Big Sur volumes and re-install it again. That should leave you with one Big Sur HD volume and one Big Sur - Data volume and then a com.update snapshot volume.
This is what I had to do on mine.
I don't know if anyone might have a better idea.