Photoshop Elements is a photo editor, but not a manager. It will probably be a great addition to the Photos.app that is coming preinstalled on your Mac. Photoshop Elements offers many tools for adding graphic composition to a photo, while the Photos.app is strictly photo centric and does not offer many tools to combine parts of different images into one.
In addition to the user guide you may want to have a look at this user tip: New to Photos? Some Considerations on How to Design your First Photos Library in Photos 5.0. I wrote it some years ago for an older system version, but it explains how the different tools in Photos relate and for what purpose they are intended.
The user guide is explaining the graphical user interface in Photos, but not the general idea, how to break it to our needs. The most important part of Photos is a relational database, that is storing and managing our photos and videos for us. It offers many predefined views of our photos automatically, we can retrieve our photos in many ways, by the date they have been taken, by the date they have ebeen adde, by the location, by the objects or people in them, and many more, all common, widely used search criteria. but Phos cannot know, why we took certain photos, or the projects they belong to, what a photos means to us. For our own, personal projects we have to design our own organisation, on top of the existing database structure. For this we can use, albums, folders, keywords, titles, captions.And we should plan our own structure in advance, once we understand how Photos is working. If you decide to use Photos, take some time to plan the design of your Photos Library or libraries.