In your original question you asked about the contents of three files:
- My Library.idz is a compressed version of My Library.idcl
By default, this library is empty. It is provided as a place to store your own data for later use with Content Center related functions. The idz file extension is the compressed version of an idcl file. If you examine the installation media for Inventor, you'll find all the libraries are downloaded as .idz files (because they are small file-size) and uncompressed by the installer to create the idcl file (huge file size).
- AI2023 Custom.idcl contains end-caps for steel sections used in Frame Generator. (It's actually a bit more useful than just end-caps, but you asked what it contains).
In your image, the two library files ASA.idcl and AUSCAP.idcl are not in the "Location of Libraries:" path indicated below:

If you know where these two files are, copy them to C:\ProgramData\Autodesk\Inventor 2023\Content Center\Libraries
If you don't know where they are, you can remove the tick to the left of the library names and save the Project file.
Peter