It could happen both way. Noted that Google is much more saturate for its ML talents, so unless you are super top-notch, some people will take it to Amazon for bigger impact. I personally know a few people, who is competitive in the industry, having Google offer/leaving Google for Amazon.
No doubt, the flow is usually Amazon -> Google/Facebook, because for mid-level engineer/scientist, money talks :), and Google/FB generally have much open/better publication policy, while Amazon is neurotically secretive. Apple is not on the picture, I think the internal engineer culture is very problematic(no common codebase, severe NIH syndrome), except for the money, it is subpar place to grow.
No doubt, the flow is usually Amazon -> Google/Facebook, because for mid-level engineer/scientist, money talks :), and Google/FB generally have much open/better publication policy, while Amazon is neurotically secretive. Apple is not on the picture, I think the internal engineer culture is very problematic(no common codebase, severe NIH syndrome), except for the money, it is subpar place to grow.
Disclaimer: Work in a ML related AWS team.