My advise would be to just start off with a single app structure. Elixir makes it very easy to refactor your app to an umbrella when, and most importantly, if that is ever needed.
You can as well (purely as an example):
can separate your code in a single app structure as well (purely as an example):
lib/my_app contexts and schemas
lib/my_app_admin admin logic
lib/my_app_web frontend
lib/my_app_api api
lib/my_app_whatever whatever
My advise would be to just start off with a single app structure. Elixir makes it very easy to refactor your app to an umbrella when, and most importantly, if that is ever needed.
You can as well (purely as an example): can separate your code in a single app structure as well (purely as an example):