2x interviews (15 mins)
1st interview: chemistry; 2nd interview: maths
Confidence in A level material; online resources
Think out loud
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.
Two 15 minute subject interviews, one after the other, I was given a map and it was fairly straightforward to find the locations.
One of them was focused on chemistry, but not knowledge based, you really just had to be able to think and interpret unseen concepts.
Second one was mainly maths, not too hard if you are good with calculus, trigonometry, graph plotting etc.
I guess the best you can do is to be good with the further maths and physics, chemistry A level materials. On top of that the more extra problems you solve the better prepared you will be. Websites like isaacphysics.org can be helpful.
You're likely to get questions you can't answer. It is good to speak your thoughts out loud, so they can help if you are misunderstanding something, or going down the wrong way. This will also save time, so you can get other questions where you can show your skills.