Time Machine Restores a Photos Library

With Time Machine you can, sometimes, drag files from the Data folder to your Mac, but that doesn't work with a Photos Library and some other special packages-- those are  rather more complicated structures.

You may be tempted toI just open one of those “Date” folders and its Data folder and navigate to the Pictures folder. You will see the Photos Libraries. (I have several.)

This is NOT a complete Photos Library package, and you can't drag it to  a Mac and have it work. You have to use the Time Machine’s Restore function to create a usable Library.

This is because Time Machine makes incremental  backups— it records only the things that have changed since the last backup. This means that making backups is really fast, so that they can be done automatically and often. But it also means that sometimes, especially with packages like a Photos Library, the backup only stores a piece of the whole, and the package has to be put together again in order to restore the entire content.  For that you need to Restore the Photos Library package.

If you open the Time Machine volume, you see "Enter Time Machine"  

which gives you this display: 

You can navigate to the date you want, select the Photos Library, and then choose "Restore." You'll be asked if you want to replace the current version or keep both. Keeping both might require a lot of storage.

Keep in mind that if you use “Optimize Storage,” then the original files that are stored in iCloud.com may not be kept on the Mac, and so Time Machine may not be backing those up. For this reason, it will be better, if possible, to not use “Optimize” on the primary Mac, even though Optimize can be very useful for saving storage on secondary devices like iPhones and iPads and additional Macs.