Images cataloged by Apple Photos are stored in an Originals folder inside the macOS package
Photos Library.photoslibrary
in the Pictures folder.
Because it is a macOS package and not a folder, it is designed to not be openable by double-clicking, and not accessible from another application’s import/open dialog box. This is because the contents inside are all managed by Apple Photos, which is tracking everything in there with its database.
There is a way to open a macOS package, but it is not recommended to do that to attempt to link to originals from a Lightroom Classic catalog. If a user or any other application accidentally alters what is in the Photos Library package, such as adding, moving, duplicating, or renaming files, the package contents would get out of sync with what the Apple Photos database expects, and Photos may start acting as if the Photos library is corrupted (e.g. you might get errors or crashes).
A possible way to be able to have a single set of photos managed by both Apple Photos and Lightroom Classic is to make Apple Photos work like Lightroom Classic. In other words, set Apple Photos to work with “referenced” files (files referenced using path links to any storage location, not just inside the Photos library). I haven’t actually tried this because I don’t use Photos often, but if you wanted to try it, you would enable the Copy Items to the Photos Library option in Photos preferences. There is a discussion of that workflow here:
How to understand referenced files in Photos library? (Apple Discussions)
If you use that method to store the original photos outside of
Photos Library.photoslibrary
then both Apple Photos and Lightroom Classic should be able to reference the same original file.