How do I sync photos to my iPhone from an external Photos library?

A couple years ago, I moved my Photos library off my main drive to an external drive. Backing up photos from my phone is no big deal. I just navigate to the library on my external, open it, and Photos displays everything as expected. But I don't know how to transfer photos from this external library to my phone. I like to manage my albums manually and only transfer selected albums, but when I use Finder to do this, I have no way to navigate to my primary library on my external. It only allows me to select Photos (which seems to be the native library on my laptop, and which doesn't have anything in it), or to select a specific folder. I'm not able to select my external library.

Thanks