Data of Volume "Macintosh - Data" still there after deleting and reinstalling OS

Dear Community,

I wanted to prepare my Macbook Pro with Big Sur for passing on within the family.

Therefore I followed all the steps to erase the hard drive and reinstall the operating system.

This has worked so far. But now it turned out that in the Finder under the volume "Macintosh - Data" is still there, but for which you have to enter the FileVault encryption password after restarting the Macbook to access it. My goal, however, was to erase everything so that the MacBook is in default factory state.

Some background info:

I had the hard drive encrypted with FileVault. Is that why it wasn't wiped/couldn't be wiped?

The main volume "Macintosh" has the file format APFS, "Macintosh - Data" has the format APFS (encrypted). Therefore it could not be deleted. Is it supposed to be like that or is the error already on that side? I read in the instructions from Apple that you should ignore this volume "Macintosh - Data" and only delete "Macintosh".

So in summary:

• How do I delete ALL data, and do I have to decrypt the volume "Macintosh - Data" before I can delete everything?
• Why does it show me "Macintosh - data" as a separate volume in the Finder? This was not the case before deleting "Macintosh" and reinstalling.

Thank you very much for your help!