How do I control which Desktop files and folders are automatically backed up to iCloud on my Mac?

In answer to your question, if you mean that you want to selectively have files on your Desktop or Documents synch with iCloud then there's no way to do what you want to do. If you put your desktop in iCloud then it's every file in the desktop. If you want to selectively synch files with iCloud then just put them in the iCloud folder in Finder's sidebar in whatever Folder hierarchy suits you. A copy of these files will stay on your Mac so you can use them even if there's no web access. If there's no iCloud folder in Finder's sidebar then Finder/Settings/Favourites and tick iCloud.

Pedantically, iCloud is a synchronization service, not a backup service. If a file is deleted from iCloud then it is deleted automatically from all your devices. That, by definition, is not a backup. If you aren't backing up your files to another disk in your possession then you haven't got a backup.

If you turn on the Desktop and Documents option of iCloud Drive then everything in the iCloud Drive folder, the Documents folder and the Desktop folders (and their subfolders) syncs with iCloud.

What I do is save things that I don't want synced to another set of folders and subfolders that I have created in my Home folder. One way to do that is to open a Finder window, use the Finder "Go" menu at the top of the screen and click Home there, then use the Finder File menu New Folder command and give the folder a name that you want.

I realize that isn't exactly what you're asking for. You may want to post to Apple via Feedback - iCloud - Apple

Dyrogerg wrote: …control what files and folders get copied to iCloud, instead of having the Mac automatically keep a copy of everthing on my desktop.

I'm not sure I see the problem. The iCloud Drive folder is a special folder on your Mac for which everything inside is synchronized with iCloud Drive at iCloud.com; everything that isn't, isn't. You get to decide.

"Desktop" is a special folder on your Mac that shows its contents on the Desktop when you're not busy with other things. (Personally, I'm always busy with other things, and I never really see the "Desktop" folder as a "Desktop.") As g_wolfman says, if the Desktop folder is in your iCloud folder, then it is synchronized with iCloud; if not, then not. Your choice.

As FoxFifth says, if you want any file synchronized, then put it in the iCloud folder.

But I'm not sure what you mean. You say "backed up to iCloud" in the title, but you say "copied to iCloud" in your post. Those are not the same, and neither one of those means Synchronize, so it's hard to know what you are asking about.

Unless you don't mean "synchronize" at all. Perhaps you mean "kept," as in "this file is kept on iCloud.com, but it is not kept on my Mac." In Sequoia you control that with a sub-menu, like this:

Again, this is your choice.

Perhaps folks have "missed the point," because the point has not been made very clear. Can you help?

macOS and iCloud don't do what you want to do. If your Desktop is synchronized to iCloud, then it is synchronized. Or it isn't.

I actually don't know of any cloud sync service that does what you want - per file synchronization. I'm not even sure how it could work properly given how computer file structures work.