The documentation states there is a page on the extension catalogs. Has that not been added yet?
The extension catalog is documented here: Extension Catalog. This was updated today, and we expect to fill in more detail in the coming days.
You can also read about the Adobe-owned extensions here: Extensions Reference.
No -- I am meaning in the documentation, not in the tool itself.
After you've selected a property, you can navigate to Extensions in the top nav, then select catalog. Is this what you're asking?