My User>Library>Containers folder has swollen to 50 GB

I contacted Mac Support to ask about this since there were no responses here. I learned that the Users>Library>Containers can be easily searched for bloated files by setting up sizes of folders in a Finder view of the relevant folder, using View>Options>Show Size, Calculate All Sizes (Save as Default). Thereby you can sort by file size and identify the uncommonly large files.

Also I learned that caches file (User>Library>Caches) that are abnormally large, most cache files can safely be deleted as they will and do recreate on the fly as needed. They don't auto empty however, but can be emptied from the app (whether Safari, Mail or other app).

Many long unused apps can be deleted (completely) and thereby also result in a large savings in space. Just be sure that ALL files are deleted when you do so as leaving old prefs, logs and caches will still overcrowd your drive.

Found out that log files were an issue as I had turned on logging in Mail and never turned it off. Cruft like logs just built up in Mail files.

Specifically those folders of concern to me were the bloated sub-folders in Containers. By analyzing for size, was able to identify the largest folders and files, got rid of most of the bloat (1+GB size files) reducing the size of the containers way down.

All in all, squeezed out about 80GB. That'll get me by. For now.

Those savings is space were of great help paring down my limited resources on the startup drive which is only 256GB, to remain at 90GB free. Highly recommend looking at the amount of space in your User Library files even if it is not a dire need to save space. Generally good to be efficient with the space you do have!