You typically need all parts (part1, part2, etc.) in the same folder before you can extract them.

So, you’ve just grabbed the first part of the files! Whether you're a seasoned horror veteran or a newcomer looking for a good scare, we want to make sure your journey into Alex’s nightmare starts without a hitch. 1. Handling the Multi-Part Archive

If you are seeing a .part1.rar file, it means the game has been split into multiple pieces to make downloading easier.