How does one add extensions?

[reshdesu]

Hello everyone,
I have just recently for curiosity and my wife added a stable-diffusion-webui with some minimal authentication with react+next+express and nginx. I was trying to follow this tutorial about object replacement and it needed new extensions to be added to webui. Has anyone done this before? How do we go about doing this?

Note: While this is been asked here, I also am asking this to claude and trying to figure out if this is documented.