What happend if my photos library is deleted by accident?

Yes you can. But be aware - iCLoud is a sync service not a backup service (see below). You should also be using time machine to be backing up your mac - including the library from the pictures folder...

iCloud is not a backup:

The problem with iCloud for photos (and files if you use iCloud drive) is it makes all the libraries across all the devices the same. While it does protect you from a drive failure - it doesn't protect you from accidental deletion. If you delete photos on your phone, they are deleted on your mac, and from iCloud - same if you delete on your mac - they'll be deleted on your phone. There have been rare cases of system failure also causing deletions (or just user error). There are also numerous examples of library corruption making it very difficult to access photos.

Every few weeks we get someone here saying "I deleted photos - how can I get them back". If it was more than 30 days ago (or they’ve been manually removed from recently deleted), and there is no other backup, the answer is "you can't"

I'm not sure that I understand your question. You seem to be aware that there is an iCloud Photos option -- see: Set up and use iCloud Photos - Apple Support

That is quite similar to the iCloud Drive (including Desktops & Document option) for your other files. Both have somewhat similar recovery options.

If you delete a file that is synced with iCloud, you have 30 days to recover them -- see: Recover deleted files on iCloud.com - Apple Support

That is similar with iCloud Photos -- see: Delete and recover photos and videos on iCloud.com - Apple Support and If you're missing photos or videos in the Photos app - Apple Support

Having said, that I use both iCloud Drive and iCloud Photos and I have never had a problem. In spite of that, I insure that I don't have the Optimize Storage option for either turned on in my Mac so that there is always a local copy there and I back my Mac up to an external drive.

Because I value my Photos Library I do the following (I don't use iCloud)

1. I have a time machine backup
2. I have a bootable clone
3. I have an external where I store *only* two copies of my iPhoto Library. I save a new one every month or so and keep the previous one.
4. The only thing I don't do is keep a copy in another location. If my work allowed it, I would store one in my office. Yer_Man stores one in his trunk.

Your photos are in iCloud if you have iCloud Photo Library turned on in your Mac. There would be no point in also adding the Photo library file to iCloud. One example -- when I set up my new Mac around 2 years ago, the way that I moved my photos to the new Mac was simply by turning on iCloud Photos on it. All of my photos automatically synced to it -- including organization albums/folders, location data etc.

I don't have any "Projects" -- they are created by 3rd party app extensions and I don't believe that they sync via iCloud but I haven't tried that.

As I noted, I installed my entire photos library on my new Mac simply by turning on iCloud Photos. The same would be true if you erased your Mac

However, if you go into the Photos app and delete photos they are also deleted from iCloud.