Erasing an external Big Sur system drive

I think your terminology is wrong, as the Container has two "Volumes", one for the OS system and one for the user account Data. This is different from "Partitions" that give distinct areas on the disk. You can have several Volumes using the same disk space.

I also had trouble getting rid of the volumes. I think you need t use the +/- icons in Disk Utility. To rid myself of the Container, I think reformatting the disk HFS+ unencrypted will do this for you. Then I did a few First Aid checks before reformatting APFS.

I find that Disk Utility does not always update it's display after taking these actions. I once ended up with three volumes showing, but, after closing/reopening Disk Utility, they disappeared and things went back to a normal single volume.

The BS installer will then make it's own changes to the disk as it requires.

I was trying to do an Erase/Install from a USB Installer, which gives basic facilities like Disk Utility and Internet Recovery options. On my first attempt, the disk had a System Volume and a Data Volume already and the installer made yet another volume, so I had three, two in use.

I individually deleted the volumes with the +/- icons before being able to delete the Container. I don't think you could reformat HFS+ right away, but someone may prove me wrong and it may be an unnecessary step anyway, once you have a single APFS volume, ready for the installer to do it's thing.