BSIPA is a mod loader based off of Illusion Plugin Architecture, designed for Beat Saber. While it retains backwards compatability, the recommended interface has been completely redesigned for a more consistent and stable installation.
Taking some inspiration from mod injectors like BepInEx, it uses a fork of Unity Doorstop to actually inject itself into the game.
The particular method of injection that BSIPA uses lets its user experience be far nicer than most others, not requiring a repatch for every game update.