Each solution is found at days/<day>/solve.py. It will read from your input file.
The template (days/template/solve.py) can read inputs in a variety of ways, passing an integer pick the input from the inp list in the code (allowing you to swap between multiple example inputs easily), and pasing -r will read from your input file. Passing nothing will read the first example input.
I use @scarvalhojr's aoc-cli tool to download inputs and submit solutions.