Welcome to Android Central! Google Photos has always had a somewhat different design philosophy than conventional gallery apps. The idea is that you have a giant pool of photos from the cloud and on your device (which would be the main Photos feed), from which you can create any number of virtual albums (which would be in the Collections section). The main advantage of this is that a single photo can be in as many albums as you'd like, without having to make copies of that photo. (In a conventional gallery app that bases an album on a separate physical directory, a photo file would have to be duplicated in order to appear in more than one album.)
There are a lot of 3rd party gallery apps out there, but you have to be careful, since this category is rife with shady devs who throw a lot of adware and other junk in.
Simple Gallery has often been recommended as a nice alternative, but the free version apparently has a ton of ads -- you'd have to buy the paid version (
Simple Gallery Pro, which is $2.99) to get rid of them.