In iPhoto, you can merge events simply by dragging one event into another. But, in Photos for Mac, all iPhoto events are now albums. Photos for Mac does not allow you to drag albums into each other to merge them. So, if you want to merge albums in Photos for Mac, then you can select all the photos within one album and drag them into the other and then delete the first album.
If you want to see a list of all your folders and albums, then enable the "Sidebar". To enable it, open "Photos" app for OS X, go to "View" tab and then select "Show Sidebar" option.
Quick Tip to ensure your Photos never go missing
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