I suspect that you are looking for EEx templates (Hexdocs).
That being said Phoenix doesn’t restrict you to using EEx templates - you can use frontend libraries, frameworks or systems like React, Angular 2, Elm, etc. as well.
Elixir applies for the most part to the backend and server-side templating.
There is ElixirScript as well but as far as I can tell for the time being that is even more niche than Elixir.