I previously installed asdf.
I setup local versions of Elixir and Erlang:
$ asdf local elixir 1.9.2
$ asdf local erlang 22.1.3
Code for each puzzle can be found in separate module files. The day 1, puzzle 1 module is named AdventOfCode2019.Day1Puzzle1 and is in the file lib/advent_of_code_2019/day_1_puzzle_1.ex.
Puzzle input files are stored in the input sub-directory and are named like day-1-puzzle-1.
For each puzzle, there is a process_input function in the respective module that will read the input file and output the puzzle answer. Some puzzles might have a function like output_answer instead.