Engineering Admissions Assessment (ENGAA), 1x interview
Technical questions, typically relating to maths and physics
Practice with teachers and practice questions
Practice papers, practice books
Read around your subject and about the subject tests
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: 1
Length of interviews: 45 mins
Online interview: Yes
Mostly technical questions where I explained my thinking and they wrote down my working on the screen. Typically with a maths/physics inter-subject theme. Not themes found in lessons but could be worked out with basic A-level knowledge.
Practice papers, practice books.