Finder says my Users folder is using up 63.96 GB in 336 items and yet there are nothing to speak of visible in any of the user folders

A quick way to see where the space is taken up...

Thanks to Bob Harris file sizes, open Applications -> Utilities -> Terminal:...

sudo ** -hx ~/| sort -h 

sudo will ask you for an Admin password that won't be echoed.

Carefully type your admin password when asked & hit enter.

Macintosh HD -> Users -> Office with the house has plenty in it. You need to use the View menu -> View Options to calculate folder sizes.

command-shift-period will also reveal invisible items. Keep in mind many invisible items are not for user's editing or deletion, as they keep the system functional.

Each user folder contains its own hidden /Library folder that contains all sorts of files & caches used by the applications installed on your Mac. That is most likely where all the files are that you say you cannot see.

You cannot see the ~/Library folder or its contents in the Finder unless you navigate directly to it by doing Go > Go to folder and enter ~/Library. But please note, ~/Library is hidden for a reason - there is almost nothing there that users should poke around in because of the risk of corrupting something there.

Among other things, Outlook, Mail and other email apps store your emails & attachments in ~/Library. If you have an iPhone or iPad, those backups are also stored there and they can take a lot of space.

The problem is your Mac simply does not have enough storage space for effective use. Get a good external SSD to increase your storage and use it for your data. If you are using, or want to use, Music, Photos or TV you can have their libraries stored on the external drive, thereby saving space on your built-in drive.

You need a minimum of 45 GB of contiguous space. Because of fragmentation of large files, that generally isn't obtained until you have freed at least 55 GB of space.

Alternatively get a hard drive with its own power source, and at least 256 GB built-in storage, to use for booting into newer OSes until have a chance to upgrade the Mac from scratch.

Take your photo and music and movie libaries and copy them to two distrinctly different external sources of media or more.

Uninstall apps that are not used or copy their installers onto these backup drives.

https://freemacsoft.net/appcleaner/ offers a nice way to drag and drop applications while booted into safe mode (shift key), to remove an application and all its components.

128 GB Macs sadly in this present day are not much better than web browsing machines. Many applications eat up more space and photo and music libraries can be a real bear.