Storage Management: What are "other volumes in containers" where do I find them? Why is my "system" using 75 GB?

One common reason for a large system storage is Time Machine's "local snapshots".

You can see which snapshots exist with the follow command in Terminal:

tmutil listlocalsnapshots /

You can also "thin" them with a command such as:

sudo tmutil thinlocalsnapshots / 10000000000 4

You will be asked for your password, type it blindly (nothing appears on screen), then press enter.

The large number is an indication of how much space to try to recover. You may try it with different values.

Other volumes in container is part of how APFS works - or, perhaps how macOS handles APFS formatted drives.

In Mac OS Extended drives, if you wanted more than one volume, you needed to partition the drive. With APFS you can have several volumes sharing the same partition.

Your mac has your system volume, and also a vm (virtual memory) volume, and two others (I can't remember what they are) all sharing the same APFS container.

You should never mess with these other volumes directly. They are used by macOS, and changing them can crash your mac and even perhaps render your system unbootable.

Ok, so that is not what is using space.

Some times there are logs that grow significantly.

Or perhaps some caches need to be cleared.

I suggest you restart in Safe Mode, and then restart normally again. This will clear system caches.

Good. It is to be expected that the VM volume will use a few GB, that is totally normal.

As long as you don't try to access these other volumes directly and use the above procedure, all is fine.