Questions about external registry cache

[mtanida]

I'm using external cache for builds in CI, and though I've read the docs, I still have questions. I'm hoping I can get some answers here:

1. When using external Registry cache, how much data are we pushing up and pulling down? I can't get a sense of how much data transfer is involved. Does Buildkit output this data? HOw can we know if we want to use compression or not if we don't even know how much data we're transferring?
2. What is in the data that we send to Registry cache?
3. What is the difference between mode=min and mode=max?
4. Is there any advantage to using GHA cache over registry cache?
5. Is there a good convention for setting external cache ref in trunk based development? For example, do people generally use the branch name for ref? The PR name?
6. Suppose i have a multi-stage bake with two images A and B (B depends on A). If i am running docker buildx bake B, it's not clear to me how to properly define the external caching. For example, should it be something like:

   docker buildx bake B \
       -- set "A.cache-from=..." \
       --set "A.cache-to=..." \
       --set "B.cache-from=..." \
       --set "B.cache-to=..."
   

Thanks!