limiting what backs up to icloud

Only items placed in the iCloud Drive folder (or desktop and documents folders if enabled) are synced with iCloud. If you do not wish a folder to be synced, do not place it in one of those locations (theHome folder is a good alternative or your downloads folder).

iCloud is NOT backup it is for convenience.

"On the cloud" is great for sharing photos, but is not a viable backup solution for everything you have. The stuff is not under your control, and is subject to sloppy handling, arbitrary changes in policy, theft, data loss, and bankruptcy of the company that holds it. It can easily take three days to restore it at ordinary Internet speeds.

If you do not have a recent local, disk-based backup, your computer is like a ticking Time bomb. You are only one disk failure away from losing EVERYTHING! Drives do not last forever. It is not a question of IF it will fail, only WHEN it will fail.

If you are using another direct-to-disk backup method that you prefer, and you currently have a recent disk-based backup, that is great. If not, you should consider using Built-in Time Machine. Take steps to acquire an external drive as soon as possible. If you buy one, a drive 2 to 3 times or larger than your boot drive is preferable for long term trouble-free operation. Do not pay extra for a drive that is fast.  (You can get by for a while with a "found" smaller drive if necessary, but it will eventually become annoying).

Attach your external drive and use

System preferences > Time machine ...

... to turn on Time Machine and specify what drive to store your Backups on.  It may ask to initialize the new drive, and that is as expected.

Time Machine may spend all afternoon making your first full backup. You can continue to do your regular work while it does this. The first Full Backup is by far the biggest backup. After that, it will work quietly and automatically in the background, without interrupting your regular work, and only save the incremental changes.

Time Machine's "claim to fame" is that it is the backup that gets done, because it does not ruin performance of the rest of the computer while doing its backup operations. You do not have to set aside a "Special Time" when you only do backups. When you need it, your Time machine Backup is much more likely to be there.

How to use Time Machine to Backup or Restore your Mac:

https://support.apple.com/en-us/ht201250

If you choose to connect your Time Machine backup drive only from time-to-time, try to do so at least every-other day, at minimum. Otherwise, it may take several hours of computation just to decide what needs to be backed up, before any files are transferred.

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Leroydouglas also wrote a nice article about Boot Clones. This might be something to think about a little further down the line.

 How to create a boot clone

https://discussions.apple.com/docs/DOC-10081