Moments in Photos are the new Events, i.e. groupings of photos sorted by date taken.
When the iPhoto Library was first migrated to Photos there is a folder created in the sidebar titled iPhoto Events and all migrated iPhoto Events (which are now Moments) are represented by an album in that folder. To open the sidebar if it's not already open use the Option+Command+S key combination.
NOTE: It's been reported by several users that if the Event albums are moved out of the iPhoto Library folder in the sidebar they may disappear. It's not widespread but several users have reported that issue. Therefore, if you want to assure that you keep those Event albums don't move them outside the iPhoto Events folder.
There's a way to simulate events in Photos.
When new photos are imported into the Photos library go to the Last Import smart album, select all the photos and use the File ➙ New Album menu option or use the key combination Command+N. Name it as desired. It will appear just above the iPhoto Events folder where you can drag it into the iPhoto Events folder
When you click on the iPhoto Events folder you'll get a simulated iPhoto Events window.
Albums and smart albums can be sorted by Title, by Date with oldest first and by Date with newest first.
If you want to merge Album A into Album B do the following:
1 - open the sidebar with the Option+Command+S key combination.
2 - with the two albums visible in the sidebar click on A.
3 - select all of the photos in A.
4 - drag them onto the Album B icon in the sidebar to add them to B.
5 - verify all of the photos are in B.
6 - delete A from the sidebar.