Engineering Admissions Assessment (ENGAA), 2x interviews
Both interviews: questions on personal statement, maths questions, then questions on concepts from the Engineering course
HE+ program, practice at explaining concepts and solutions
Lots of timed past papers
Remember Oxbridge isn't everything
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: Engineering Admissions Assessment (ENGAA)
Number of interviews: 2
Time between interviews: 20 min
Length of interviews: 30 min
Online interview: Yes
Surprisingly I was asked about my personal statement in both interviews, which I wasn't told to expect for a STEM application. This was geared towards the books I referenced in what at HE+ was dubbed "learning chains" where something piques your interest, so that leads you to listen to something about x, which in turn makes you want to read something about y. I remember being told to be critical (in its evaluative definition) in the delivery of these—why they were interesting, what they did for me, and how. This is in opposition to just simply saying "I read this, I built that, I learned this". Why did you do them, what did you get out of it?
On the more functional aspects of the interviews, I kid you not, it was very straightforward (mildly challenging but not too bad) A-level maths questions that just had a fair bit of reasoning on what to do next (like it took 5-10 min). Then there were questions that weren't as straightforward, and didn't have a "correct" answer that ended a bit more open-ended but were still mathy and touched on concepts solidified in first-year engineering. I don't want to go into too much detail with the specifics, as I doubt they'll come up again, so revising those specific questions would be pointless.
The key takeaway I suppose is verbalizing your thoughts, saying why you're struggling, and trying not to close down when something feels hard.
Little help from my (sixth form) college, but I did go on the HE+ program which helped with competitive applications stuff, and what sort of attitude you need to have—emphasizing that the answer isn't the important part, it's the thought process and the verbalizing of your thought processes (I remember being told that the interviewers want to know if you're going to be receptive to discussion and talking out your problems and ways of thinking and how to develop them etc). I didn't have any practice interviews, but I practised explaining concepts and solutions to problems to my mum, even if she didn't fully understand. If you don't know the answer, say what you do know and why you're struggling—this is a key skill in
Loads of past papers, emphasis on mental math skills and quickness (which I lost immediately after), and getting comfortable working to the time pressure and not answering everything
Remember Oxbridge isn't everything, and not getting in doesn't invalidate your hard work, ability, interest, and ambition—applying already shows you love what you do!