How does gmail classify emails into a specific category?

Question

gmail

By default, the Gmail interface has several basic categories:
Unsorted, Social, Promotions, Alerts.
I contacted Google technical support and they answered me that the algorithm and selection criteria are confidential information, and that this can be found out for an ordinary user only by experience.
Perhaps someone knows how Google determines the class of the letter?
I have a rough idea, but very biased:
1) by the domain and its behavior on the Internet
2) by the title and body of the letter (presence of pictures, text semantics)
I would like some specifics.
This is necessary for practical purposes, notification letters fall into Promotions, but you need to Alerts.

Answer 1

Artificial intelligence, roughly speaking, Google sat and taught AI, as a result, this is what happened.

Answer 2

Just the presence of certain words in the text. Like spam classification, but for categories.
See Naive Bayes classifier.