You access it by opening Photos, then open Settings/Preferences and on the General tab look at Library Location. Note what your current default library is and where it is stored. You can change the library from whatever it is now to the iPhone one and Photos will open it for you so you can see what is in it. You can also do similar by holding down "Option" then click on the Photos app to launch it. You then get a dialog to pick the library you want to open. Again, note the name and location of your current library so you can get back to it without a lot of hassle.
Once you have opened the iPhone library, look at the images to make sure you don't need any of them. Once you are content that you don't need it you can close Photos, then relaunch with Option held down and point back to your default library. Once that is done, you can decide what to do with the iPhone photos library file. If you are 100% certain it isn't being synced to your iPhone, you can delete it. Personally I'd make a copy of it before I did that, but you do whatever.
BTW, you called it a directory, we just tried to correct that inaccurate statement. Then you responded "I know it is a library." But you called it a directory. Tech terms are very precisely defined, and it helps a lot to be precise in your posts to avoid confusion both for us and for others who may see this thread later on when trying to solve their own problems by searching.