Confused about DSA and evidence?

So firstly what evidence do I need to send? I have autism and was diagnosed at age 19, I still have the ADI-R report used to diagnose me. However although it goes into some of my difficulties (or what my mother reported anyway, which wasn't all of it) it doesn't really say how it effects studying or learning. If I need to get a letter from the doctor how do I go about doing it, do I make a normal appointment and take in the form or do I ring up and say I need the doctor to fill in a form.

This is mostly for my curiosity (since I was diagnosed at 19) but it mentioned about having to have a new diagnosis for some disabilities if it was originally done under the age of 16, is this the case for autism? Do they have to have a re diagnosis or can it just be a doctor's letter confirming they still have x disability.

What's the assessment like? I'm in the process/war of trying to get PIP, I'm worried it's a similar thing where they are completely working against me or think I can manage fine. What sort of things do they ask in the assessment? Do they tell me on the day what I will be getting or do I have to wait for a letter to tell me.

For other people who have ASD, what do/did you get? I'm doing computer science if that makes a difference.