Elixir projects with application under Phoenix umbrella (DDD)

Hi, I created a Phoenix project which uses umbrella structure, but I’m confused because I have 3 or 4 projects under it that are created with mix new NAME --sup and they have own application ex file.

Now how the Phoenix umbrella project knows it at start, some services should be started under each application file when I start phoenix project as main.

Do I create a main application file in my project and add all the sub project application module ?