Other volumes in container 215GB

You can create an installer on a usb flash drive, or install using Internet Recovery.

I would only use Internet Recovery as a last resort.

Here are some instructions on how to create an installer on a flash drive and how to do a clean install:

https://www.macrumors.com/how-to/perform-clean-install-macos-catalina/

Did you make a backup?

Remember, you are about to erase the whole drive!

Sure, if you have a backup, you can use recovery.

Or you could perhaps try to delete that extra volume and use the recovered space to download the installer.

Control-click that greyed out volume, and choose "Delete APFS Volume..."

You have two instances of "Macintosh HD - Data", where there would normally be one.

Moreover, one has some 13GB whereas the other is much larger.

It seems that this can happen to people who have reverted to Mojave and then upgraded again to Catalina...

I am a bit concerned with the fact the "other" Macintosh HD - Data seems to have a lot more data in it than the one you are using. Does the 13GB seem ok compared with your own data?

It could be fine, but my relatively small Photos Library is all of 7.5GB... my Music Library alone is over 13GB...

If you select your home folder in Finder and press command-I, how much space does it says it uses?

In Disk Utility, select View->Show All Devices; then select the container, and tell us what you see.

Or, better yet, post a screenshot like this (anonymizing any private information):

Then I can only suggest that you back up your data, reformat the whole drive and do a clean install.

This probably got into your hands already in this "strange" state.

By deleting that extra volume, it is probable that everything will be all right, but still I'd recommend erasing and installing.

That is what the previous owner should have done in the first place.

Ok, then I suggest you try the "Delete APFS Volume..." in Disk Utility.

Make sure you delete the big one, that is greyed out, and not the one that has your data.