how do I know what "Other Files & Folders" contains? There are no details, but its 96gb, my username has a total of 110gb, so its substantial, just no idea what it is and if I should transfer it?
Yeah, I would say 96 GB is significant! I have never had it that high.
Unfortunately I don’t know of any way to find out what migration assistant has put in that category. Presumably they are files outside of your /Users folder. One example could be the contents of /usr/local, for example. That is where Brew used to install all of its files. (I think they are now under /opt though?)
Try to think of anything you might have installed or saved outside of the usual folders. Did you create a Steam directory storing a lot of games? Do you import photos and videos to a non-standard location?
Perhaps others can think of typical possibilities…
EDIT: I wonder if the contents of your ~/Library folder is categorized as Other. Some apps can store a lot of files in there. For example steam does by default, I believe.
I guess I can skip it and transfer everything else, then deal with it if I notice things are missing? No idea why I ordered 256GB, and past the return window now. maybe I need an external hard drive?
On your Mac, see a visual representation of the storage space currently being used by apps and files, and see how much space is available.
support.apple.com
It's not anything within your Home ~ folder or other Users.
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.
A few key items are fonts and "documents & files from apps" It appears it is basically "miscellaneous" that doesn't nicely associate with the other couple of options. Some say it's fine to leave it unchecked and others imply that some of it is needed. A recurring definition says it is other files in the ROOT folder, which is where apps would store their own supporting files & similar but also where needless cache files, etc are stored that wouldn't need to transfer over.
Note that both of the above links are quoting the documentation for the System Information.app. That definition may or may not be the same for Migration Assistant (judging by Apple’s consistency the last few years, I’d guess probably not 😏).
The help document for Migration Assistant doesn’t give a clue as to what gets included in “Other Files and Folders”: https://support.apple.com/en-us/102613
That’s what I would do. Especially if you can keep the old Mac around for a while, or better yet, have the old Mac fully backed up so that you can retrieve files if you need them later. 96 GB is a lot of data.