Interesting and I don't know but if I were to guess I would say that it looks at the type of messages/senders that you interact with and uses that as one of its signals. If the only thing you get sent to your gmail is spam or no-reply messages and you never reply to any of it I suppose it has nothing to go on and just assumes that it is all weighted equally.
But like I say, that's 100% guesswork, based on the 50 emails in my inbox being not-spam and 16/18 emails in my spam folder being spam (and thanks for prompting me to check, btw!)
But it clearly does have something to go on - the gmail messages of millions of other users! And as I noted in an edit, ChatGPT can fairly easily tell them apart, even just from the subject line/first paragraph (and even explains why it believes they are or aren't spam).
But isn't google's whole schtick about creating a personal profile just about you? Maybe they think you are in the market for some offshore v!agra or would like to get in on the ground floor of some crypt0 investment opportunity...
But like I say, that's 100% guesswork, based on the 50 emails in my inbox being not-spam and 16/18 emails in my spam folder being spam (and thanks for prompting me to check, btw!)