The problem is that a technical interviewer can ask literally anything technical. That means you have to be able to answer literally anything. That means that you have to know (or be able to figure out) everything. That's really hard to prepare for.

The best way to prepare is to actually learn as much as you can - not as "interview preparation", but as actual knowledge. Then, as specific interview skills, you also need to learn to think under pressure, to communicate what you're thinking well, and to gracefully and smoothly say that you don't know, but you could figure it out with a bit of time and Google.