Difficult storage problem. So, I moved all documents to an SD card in order to erase my MacBook storage. Somehow, still, I only have 20GB free storage on my computer, while 76GB is qualified as "Other Volumes". In both, Macintosh HD and - data. Idea?

Many times people find the storage space is being consumed by hidden APFS snapshots used by backup software such as Time Machine....even third party backup software utilize these AFPS snapshots. Usually these backup snapshots are automatically deleted once the backup has been fully transferred to external media, but many times one snapshot may be retained on the internal boot drive, however, sometimes multiple snapshots may be retained depending on the configuration of the backup software. You can use the instructions in the following Apple article to view & delete APFS snapshots:

View APFS snapshots in Disk Utility on Mac - Apple Support

Also, if macOS has been reinstalled, many times people do so incorrectly so they end up with two "Data" volumes so all their data is stored twice on the boot drive. This can usually be seen if you look in Disk Utility to see if there is more than one "Data" volume listed. Many times you may see something like "Macintosh HD - Data" and "Macintosh HD - Data - Data". I would suspect this latter issue in your case.

Uninstall CleanMyMac by following the developer's instructions. CMM has been implicated in so many macOS issues that many consider CMM a form of malware itself. Multiple people have told us they have used CMM for years without any problems, but then one day CMM is the cause of their problem. Usually people encounter issues much sooner.

After uninstalling CMM, also uninstall any other anti-virus apps, cleaning/optimizer apps, and third party security software by following the developer's instructions since none of them are needed on a Mac and will usually cause more problems than they solve. You may want to review this excellent article written by a respected forum contributor as it contains a lot of good information and tips:

Effective defenses against malware and other threats - Apple Community

You can run the third party app EtreCheck and post the report here so we can review it for clues. Plus it may reveal whether the developer's uninstaller actually did their job to fully remove all traces of their apps. Many times CMM does not get fully uninstalled. Plus this app will also show us the drive layout which can also reveal whether you may have more than one Data volume from improperly reinstalling macOS.

mayusch wrote:

As the entire report exceeds the 5000 character limit, I've tried to include it as "Additional Text".

That is the proper method of posting the report.

However I do have the option to erase the Macintosh HD - Data (disk1s1), I am just a bit scared to delete it as it is encrypted.

From the EtreCheck report that 72GB "Macintosh HD - Data" volume marked as "disk1s1" can be safely deleted if you do not need any data within it because you have a backup. This volume is shown mounted at "/Volumes" which means it is not currently being used by the currently booted OS so you are not going to break macOS by deleting it. Whether you need to retrieve any data from the old Data volume is another matter which only you can decide.

disk1s1 - Macintosh HD - Data

Filesystem: APFS

Mount point: /Volumes/Macintosh HD - Data

Encrypted

Used: 72.01 GB

If you look closely, the other "Macintosh HD - Data" volume marked as "disk1s5" is shown mounted at "/System/Volumes" which indicates it is an important part of the currently booted OS.

disk1s5 - Macintosh HD - Data [APFS Virtual drive]

Filesystem: APFS

Mount point: /System/Volumes/Data

Used: 3.79 GB

The nuclear option would be to back up everything to an external SSD, boot into Recovery mode and clean reinstall your OS, then migrate your setting, data and apps back from storage.

AFAIK you should be able to view APFS snapshots when booting into Big Sur. Within Disk Utility you do need to first click on the "Data" volume on the left pane of Disk Utility before the option "Show APFS Snapshots" appears on the View menu (on the menu bar) of Disk Utility. It is possible you don't have any snapshots at this time.

You should be able to delete that APFS volume assuming it is not the one you are booting into.

I don't see the EtreCheck report.