where is the photos library stored in Catalina?

By default the Photos Library should still be in your Pictures folder.

You can see in Photos, where your current Photos Library is stored.

• Launch Photos, if it is not running.
• Open the Photos > Preferences > General. The top row is showing the location of the library. Below it is a button "Show in Finder".

To see all libraries, that Photos can find, quit Photos, then launch it again while holding down the options key ⌥. Release the key, when the library chooser dialog appears.

The dialog will show you a list of available libraries. One of them should be marked System Photo Library.

Does any of these libraries hold the missing pictures?

The folder users/shared/previsously relocated items/security/pictures is created during a system upgrade. The new system is moving the incompatible system extensions and frameworks there. It is extremely strange, that photo library has been dumped there.

I am glad you found your library.

In addition to the incompatible system extensions and frameworks léonie mentioned, files could be in incompatible locations! Catalina won't let you store your own files and folders outside your personal data area. If Catalina installation finds user files or user folders in one another location, they will be moved into Relocated Items.

A common place where some users stored user data that is no longer allowed is at the root of your system volume. (I got caught by that one.)

Ouch. I always had the iPhoto/Photos library at Users/Shared/ so rest of family could access it (now it is too big for my internal disk so it is on the external). Odd that it was relocated by Catalina installer. I wonder how many Apple users got bitten by that oversight.

I did not think that /Users/Shared would be handled that way, but I have heard that it is no longer a safe place for Photos libraries. The only apparently safe place to store shared Photos libraries has been on a separate volume with file permissions ignored (only if you never have two users logged in at once), and even that may no longer be safe under Catalina.