Multiple (but different) Photos libraries - how to combine?

Hello! Today I tried to consolidate all of my photos into the Photos app on my Macbook. I already had 4+ years worth of photos in my existing Photos Library, but I also had a lot of others still saved in my old iPhoto library, plus some in Dropbox and some stray very old ones just saved in random folders. I followed instructions online for exporting my iPhotos library to Photos. But in doing so, I seem to have created a new Photos Library, and now the latest 4 years worth of pictures aren't anywhere to be seen in Photos anymore.

In the Finder, there's now two files next to each other, Photos Library (which I presume is everything that was in Photos before today, so I'm not concerned I've somehow lost them) and Photos Library 2 (which seems to be everything I added today that I can view in Photos currently.) I'd really like to combine these two so everything's organized nicely in Photos, but I'm afraid of taking a wrong step that will delete or make inaccessible that original Photos Library. (I haven't deleted the old iPhoto library from its original location yet, so I'm less concerned about those pictures!) I'd be grateful for any tips or guidance!!

[Please note that I am not particularly tech savvy, so I'm a little afraid to ask this for fear of lots of jargon. And in case this comes up, I have chosen not to use iCloud, for affordability/space reasons. I back up my devices on an external hard drive instead.]