Online courses, I don’t know, but resources we do have plenty:
On OTP:

• Designing for Scalability with Erlang/OTP [Book]
• part 2 of my fav elixir book by Sasa Juric:
  Elixir in Action, Second Edition [Book]
• erlang in anger and pretty much anything by Fred Hebert:
  https://ferd.ca/
• talks from erlang solutions, code beam and many more.

On functional programming:

• CS 3110 by Cornell, ocaml but useful :
  http://www.cs.cornell.edu/courses/cs3110/2020sp/textbook/
• the scala series by epfl (scala obviously)
• the funmooc series by INRIA if you’re not bothered by the strong French accent (ocaml, but applies)

Hope this helps.
Oh and I almost forgot: popular elixir libraries/frameworks have VERY readable code, and thanks to HexDocs it’s literally one click away from the doc, so read it as well.

I don’t know if I should mention this here but Erlang Solutions has a number of courses and tutorials some of which can go quite deep.

Apart from the ones mentioned above, I find that this talk by Sasa Juric explains on a very concise manner the core of OTP:

Also, if the idea is to use the knowledge on production, I find that the book Designing Elixir Systems With OTP by James Edward Gray II & Bruce Tate covers not only what is OTP but also gives an approach as to how to design an application with that in mind.

Though it is a book and not a course, the narrative guides you through the construction of an example project while explaining the concepts, so it is also pretty hands on if you follow it that way.

There are also some small videos on Elixir Casts about GenServers, but those are not that “hardcore” I’d say, but still are very good and well explained summaries on different Elixir topics.

Hope that helps!