Working in After Effects means working with projects. After Effects projects are similar to Premiere projects - you import files, store them in the project panel and work on them in the timeline.
After Effects projects are saved with the .aep extension (short for After Effects Project).
There are a few things to be aware of when working with .aep files...
Only one project can be open at a time. If you open a new project the current one will be closed. You can however import a .aep file as an asset. All the assets of the imported project are included and available for the main project.