Maybe this is a nuance that makes it “better” but I’m 99% sure that Google login means if so visit a site that supports it, Google will share my email with that site even if I don’t explicitly speaking login. Maybe there’s a workaround these businesses are using to get it? But either way my inbox is littered with emails I didn’t sign up for,

Ah, you meant "sharing my e-mail addresses". Not the actual emails (the email contents) themselves.

Google may not share the emails themselves but they for sure read those emails and sell data analysis of those emails to third parties. That's why other companies like amazon never sends emails with lists of what you bought.