where can I see the files stored in "Other" ?

How to free up storage space on your Mac and purchase and external drive and move as much of personal data files, pics, videos, music etc off the internal drive to the external drive.

See used and available storage space on your Mac

• 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.

I recently updated to Mac OS Big Sur, OS 11.2.3. "Other" is taking up 156 gigabytes of my storage. When I reduced the files in documents and movies to increase space, the storage space taken by "other" INCREASED. So if macOS is managing 2/3 of my storage...WHY?

Telling me I can't manage a category as large as that is not helpful. Saying "the category varies in size depending on the current state of your Mac" is also not helpful. How do I find out "the current state of [my] Mac"? And how can I change this "current state" to reduce "other" contents that are eating up storage?

Use WhatSize or OmniDiskSweeper or Daisy Disk to determine exactly what files and folders are using the most space on your Mac. Some of these have free versions available. Be careful about deleting files: generally, files inside your user directory folders (Documents, Pictures, Music, Movies ... etc. are ok to delete if you have safe copies of them in an external drive or somewhere else. If folders or files inside your Library within your home directory are taking up a lot of space, be careful, those can contain backups of your iPhone or iPad, or can contain your emails, etc.

~Library/Application Support/MobileSync is a directory to check -- it can be very large if you back up one or more iPhones to your Mac.

Do not shoot the messenger - Apple has made the statement quoted earlier.

To better serve your purposes and to be constructive - send your Feedback to Apple.