I just really want to see where the numbers come from.

You know people self hosting email, I know people self hosting email. But that is certainly not the case for the vast, vast majority of individuals. For businesses, I have seen Exchange take over what used to be smaller hosts, and Google has broken into the small/medium business world as well. I have searched and searched and found nothing, but I don’t see why it should be so hard to do. Obtain a list of email addresses from some data breach (I dunno how but I’m sure security researchers do it all the time) then check their DNS to see what proportion point at big tech. My gut feel is that it’s a large proportion, but maybe that’s just the corner I work in.