What doesn't back up to iCloud?

When you delete a username, what isn't backed up to iCloud if you have everything set to sync?

I can find a list of what does back up to iCloud, but not what doesn't.

What I've come up with so far:

1. Downloads Folder
2. Application Data - Some apps will backup to iCloud. I've also noticed some put files in "On My Mac" which are files that which do not sync to iCloud.

For my situation I am trying to delete a username but just seeing what I want to do to make sure I've transferred all my data that I want off this username. I've checked Downloads and I think (hope haha) I've made sure any app data I care about the most is backed up... but that's tough to truly know.

Note - I am still on MacOS 12.6 b/c of tech issues. I've had a ton of calls with apple support literally over the last couple years with a variety of issues that have to do with what is taking up all my storage space. About half of it is often taken up by other. Yes I likely will need to do a fresh re-install of the operating system, but even with that they haven't promised that will solve the issues because they still have it classified as an "emerging issue" which has been the case since at least late last year.

For context, I have multiple usernames synced on the same iCloud (what was recommended by apple to do in my situation). They may all be having issues downloading too much stuff off of iCloud, but the username I use for work most clearly does with iMessage (I use different usernames basically like focus modes. It worked great for a year or two and then came crashing down because of these issues. I used it before focus modes came out, but I also find those to be barely effective due to really poor design from my point of view. If someone here wants tips on better design I'd be happy to share ideas). I didn't use to use iMessage on that account but once I set it up for whatever reason it's wanted to download all the messages off iCloud (for context it's downloaded close to 50 gb of my message data, nearly all of it, while in my main username it's downloaded I think less than 5 gb.) Yes of course deleting stuff is something I could do and do do, but it's a ton of a manual work and why I pay for iCloud storage to automatically offload data like this.

Also note it's chewed up so much data in this process (close to my almost 80 gb of free hard drive space before this iMessage issue) that I can't even get backups to work properly. So I'm going to delete this username but just seeing what I want to do to make sure I've transferred all my data off of it. I do think iCloud did get one full sync in before I really stopped using it... but not 100% sure. I have time machine backups thankfully and also have iDrive (3rd party backup service) from forever ago but don't really pay attention to what the heck it does anymore (another thing on my to do list haha).

Here's the closest two relevant support articles I've found. But I find them both to be lacking for my situation.

What does iCloud back up? - Apple Support

Transfer your information to Mac from another computer or device - Apple Support

Thanks for any help you may be able to provide :)