Disk Space not freeing up + weird volumes appearing in Disk Utility

In Catalina - Apple brought in a Read Only Volume called Macintosh HD that contains the Operating System and is protected from unauthorized changes being made to the Os. The second Volume is Macintosh HD-Data, that is where the User Data is stored.

The OS itself sees the Two Volumes as One and this is by Apple Design.

Note: Volumes are not like Drives or Partitions as the Volume can expand and contract as needed and without the older method of have to Reformat the drive into Partitions.

Now the Free Space issue - one can try the Rebuild the Spotlight index on your Mac and check afterwards if there has been any improvements.

What is “Other” storage on a Mac, and how can I clean it out?

Free up storage space on your Mac

OmniDiskSweeper - safe to use

See used and available storage space on your Mac

Then the Final Word from Apple on managing the " Other " category

• Other: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.