"system photo library" designation: move back from external SSD to Mac

I'm a longtime iphone user new to Mac, and want to undo a rookie mistake: I recently bought an older MBP retina 13 with 128 gb storage. It was connected to my 2tb icloud account. I bought a 1tb SSD with intention of moving my photos system photo library to the SSD to free up space on the Mac. I followed the instructions to do this, and all went well: I copied the photo library over to the SSD, disconnected the photo library from icloud, and followed the instructions to make my newly copied SSD photo library my "system photo library." BUT...when I went to connect this newly-designated photo system library to icloud, I got the message that photos will only work with Mac OS Extended and APFS. I had left my external SSD formatted to EXFAT not realizing that this format does not support photos or icloud. So, what I want to do is make my photos library on my mac (which I did not delete) the system library, so I can format the SSD in APFS and do the whole procedure over again. But, (again, remember: new to Mac!) I now cannot find the photos library on the Mac hard drive in order to use preferences in Photos to make it the system photo library. I could just start all over and wipe the Mac, but would like to avoid that. What am I missing?