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: 4
Time between interviews: About 12 hours (they took place over 4 days)
Length of interviews: Hour-ish
Online interview: No
None of my interviews brought up personal statements - they said they were only assessing academic ability and there wasn’t time to go through the personal statement. One of the interviews involved answering some questions similar to the admissions test I took, one involved discussing a philosophical question and arguing both sides to try and think critically about it and one involved analysing a piece of philosophical text and talking about it in the interview and what it meant. Some of the interviews felt very casual and friendly, and some felt more formal. They said we could wear whatever clothes made us comfortable which was nice because they weren’t going to judge us on any ‘formal outfits’ we’d brought
I was told not to worry about interview revision because it wasn’t a test of knowledge (for the course I was going for) so I just did a bit of maths Wikipedia browsing the day before
Lots of practice papers (even though the syllabus changed the year I took it so they weren’t entirely accurate), some further maths revision on a few topics (I didn’t take further maths), YouTube maths tutorials
There’s a slightly unpleasant phrase, “verbal diarrhoea”, meaning to just say anything and everything that’s on your mind, and I’d definitely recommend doing that in the context of interviews - they can’t read your mind so you need to say it out loud! Talk about what you think the problem might involve, your thought process, whether it reminds you of a similar problem you’ve done before, if you have a gut instinct on how to solve it, if you think drawing a diagram might help make sense of it more, just anything that comes to mind in relation to a question you’ve been asked. Not only will this show the interviewers that you can think through a problem in different ways, but they’ll often give away if you’ve got the right idea and lead you towards that, and together you then end up with the right answer, even though you weren’t sure how to go about it!