4 x 40 min interviews
Graph sketching and number theory, logic
Practice interview, reading articles and books and some logic exercises
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: Two on the same day, two in the next week
Length of interviews: 40 minutes
Online interview: No
For maths, it was just general problem solving. Graph sketching and number theory seemed to be most common. Also proofs which would be guided by the tutors. Overall quite relaxing. For philosophy, I had some reading before the actual interview, mainly to test your understanding and whether you can comment on arguments. The logic exercises are easy if you do some practice.
For philosophy, just reading articles and books and some logic exercises. For maths, I had a practice interview with my teacher at school
Past MAT and STEP papers
Preparation is really useful. Having a really good understanding of your subjects means you won't get surprised by the questions they throw at you. Once that's done, just relax into the interviews -- it's fun talking to people who share your interests and are obviously really good at them!