Remember this advice isn't official. There is no guarantee it will reflect your experience because university applications can change between years. Check the official Cambridge and Oxford websites for more accurate information on this year's application format and the required tests.
Also, someone else's experience may not reflect your own. Most interviews are more like conversations than tests and like, any conversation, they are quite interactive.
Test taken: Mathematics Admissions Test (MAT)
Number of interviews: 3
Time between interviews: One day.
Length of interviews: 15 minutes (all of them)
Online interview: No
Each interview started with a question related to the personal statement and it should be pretty basic - just to check that you were honest in the essay. In the first interview they gave me around 3 different problems related to the things I talked about in my statement and we went deeper and deeper into them until I couldn't easily answer their questions on the spot anymore. I basically got a load of follow-up questions discussing different sides of the initial question. The other two interviews had a higher variety of questions and less follow-up ones. The questions were completely academic and related to my background in maths and computer sceince (about which I talked in my statement).
They will ask you about the things you wrote in your essay so you should maybe repeat the basics of the books/materials/science things you wrote about. Also, you might want to try explaining your thoughts in English to a friend (or a rubber duck if that feels too cringey). Keep an open mind during the interview and don't think about the mistakes you made/you think you might have made; if you stay calm and keep your thoughts clear things should be fine. If you think you just tanked an interview - don't worry;
I did the previous MATs and timed myself.
Be natural in your personal statement and during your interview - they should be normal discussions. Also, do whatever you want and do your best to feel comfortable during your stay in Oxford/Cambridge.