I've found that turning on the sidebar has helped me adjust to Photos much more quickly.
Every import dumps photos into the All Photos album as well as the "Last Import" view. You have to create each album by choice and there are now no events that you can control directly.
You can choose the "summarize photos" option or uncheck it to get more or less detail when browsing by time, but Photos makes decisions based on the date metadata with each image.
Collections seem to replace events, but again - they are generated algorithmically and not something you can edit. Best case, change the metadata and export the original photos you wish to "edit or change" and then delete them. By re-importing the original images, you can perhaps influence the collection and ordering by algorithm, but the algorithms themselves remain the same.