Hi there! Firstly welcome to the AI speedrunning community! Hope you're having fun with the game :) As Cold said, the technique to getting down the stairs is called psychorunning. Here's a link to a good explanation of it with keyboard inputs so you can see the rhythm for when to press crouch: Left turns can be tricky so require quite a bit of practice to get used to.

Also ignore the video title as any% is now a lot different to what it was when that guide was made. The guide is essentially an all missions no cc tutorial nowadays. If you have further questions then feel free to ask here or in the discord!

go fast