IMO anything that encourages you to learn the stdlib and common packages (Ecto, etc) is a good place to start. The important thing is to write code, write tests, and debug lots of errors.

Small problems like those in Advent of Code can be a good way to practice manipulating data and breaking apart complicated problems into smaller steps.

For larger problems, there are already some books that specifically walk through building things in Phoenix - but you could also take most any other language’s web app examples and build those in Phoenix (especially useful if you already know another set of tools).

The important thing isn’t to memorize all the APIs (nobody does that), it’s to have enough exposure to them that you can remember to go look them up. For instance, you won’t get a ton of value from memorizing every nuance of what Ecto.Changeset.unique_constraint does, but you’ll get a LOT from remembering it exists and is in Ecto.Changeset.

You should learn functional programming as Elixir presents it. The official website’s guides are amazing for that.

But your best quality as a junior is the ability to ingest educational material and generally learn quickly.