How IMAP works under the hood

Of course these days the mega-corp walled garden email providers don't really follow standards like IMAP. IMAP will not work with, say, Google's gmail or Microsoft office365, or AT&T ISP email, etc, etc. They have each implemented their own proprietery out-of-band authentication system that only works over HTTPS using the OAuth2.0 toolkit to build it. Any email client that does not explicitly design for each particular OAuth2.0 implementation (each megacorp's is slightly different) will not be able to connect over IMAP (unless they login via HTTPS using a web browser and set up "app passwords" for google, or similar for others).

Struggling to think of a way in which "IMAP will not work with gmail". Please explain.

It can, but it does require doing a lot of Google-specific things (set up a google cloud account, create a consent screen, get a security review, justify your usage of the IMAP API instead of the web APIs to them, find the right scopes, etc) or instruct users to go through multiple screens in their google settings to create an app password.

Google really doesn't want you to use IMAP. They're trying to push everyone to their neutered web apis instead.

You seem to be taking the perspective of an application developer or something like that? Certainly for users all they need to do is roll in with their favorite IMAP client and use it. All of what you said applies not at all to users.