You are right to say that there are generally two directions it can go in
1. Reflective questions
2. Typical competency and motivational questions similar to anything you could get asked in a VS interview.
For reflective style interviews, the best thing you can do to prepare is having a very comprehensive work diary throughout the scheme. So keeping track of all the work you’ve done during the scheme and all the conversations you’ve had with people. You would need to draw on all this to answer questions like
1. What work have you completed? - Pick a few highlights and expand on why you found them interesting
2. Can you give me some more background on the task? - This is the where you’d need to demonstrate that you understood the wider context, what the client wanted, the role your piece of work would have had
2. What task was the most challenging and why? - This is why I always recommend asking for feedback on tasks because it makes answering this question easier.
3. What task did you find the most boring and why?
4. What do you think could have gone better during the week? What areas could you improve on?
5. How did working with your teammates go on the group task? Were there any issues in working together, how did you overcome them?
6. How did you find adapting to working from home? - Only including this because I remember being asked this and being pushed on my answer a lot and feeling very confused afterwards as to why. I’d suggest highlighting the good and the bad and framing your answer in a way that shows you’re happy to work in both environment.
7. How did you enjoy your time at the firm? - Think of this as a why this firm question but make sure your answer emphasises the things you’ve learnt about the firm while on the vac. You could also discuss stereotypes about the firm and why you don’t think it’s true. “I heard XYZ things about the firm before the vac, and I was pleasantly surprised that ...”
For motivational/competency based interviews, any prep you’ve already done for a VS AC will be more than enough. Similar to what I’ve said above, just make sure to update your why this firm answer so it includes experiences from your VS. Be prepared to be pushed harder on your answers as well. I don’t think the kind of questions you get asked are tougher, it’s just that they expect more from you and your answers. To give you an example, one firm I interviewed at asked me a straightforward question on why commercial law. They then spent 15 minutes pushing me on this answer, in ways I’d never encountered in a VS interview.
To answer your question on American firms specifically, the TC interview I had with an American firm was focused on motivational and competency questions. Take this with a grain of salt but I think that because schemes are virtual now, you don't get involved in as much live work and both you and the firm have less time to assess whether it is a right fit . This means that a lot more emphasis is placed on exit interviews as an additional form of assessment and an additional way to assess the fit and any other qualities. So I think that motivational/competency interviews are more likely than purely reflective ones. The one interview I had which was purely reflective was with a firm that did get us involved in a lot of live work so I think there was more scope to ask the questions I listed above.
Hope that helps!