how do I merge volumes in big Sur?

I'd backup, delete and restore the Bootcamp partition to the end of that pie (does a new partition always go to the end?). But is it possible to clone Bootcamp -- in the distant past I successfully tested Winclone for this but I prefer VMware VMs although it is not as fast as native boot.

You can’t because the Bootcamp partition is in the way. On that pie chart, the storage runs clockwise from top. You can only merge the free space into the immediately preceding section in the counterclockwise direction.

You would have to remove the BootCamp partition (using bootcamp assistant).