Big companies put candidates into an ATS (applicant tracking system). You've unknowingly put yourself into such a database. Even if you only have them your email, they can basically get the equivalent of your resume from a variety of sources. They also know you're growing your machine learning skills so they can match you to machine learning jobs.

Now, Amazon's army of recruiters are all running queries in their ATS to find candidates to reach out to. There is likely little to no coordination between these recruiters which is why you're getting multiple people reaching out.