How?
I'm pretty careful about what I share with various services. Facebook (Instagram, etc) are all silo'd off to separate access points, phone is devoid of any of them, you get the gist. The only thing I didn't mind was Google, since I was paying for it, and it's easy to think that paying for it means it'll work how you expect it to and your privacy is intact.
Thus I'd keep Google apps signed in to the account, and I'd keep a tab open with Gmail all day, so short of incognito tabs, I was almost always signed into it.
Your _email_ isn't scanned, but Google doesn't even do that for free email accounts since..., what, 2014? Most people who decry the whole Gmail reading emails angle don't seem to understand this. They still track you as you move around for advertising purposes, though, and that was enough to make me just not want a Google account.
Siloing social media probably reduced my targeted advertising by 20%, and the absolute biggest reduction I found in my life was by getting rid of Google. It's honestly jaw dropping how much tailored content you get shoved at you that "waking up" from it can blow your mind (read: less depression, fomo, and so on, which more people seem to be catching on to).
YMMV though. I don't hate advertising and think it certainly has a place in society, and some people find it useful/fine/acceptable. Just noting my experiences.
Firefox containers solved[1] this for me. They are probably the biggest privacy boost I've felt for a decade. I've got three separate Google containers (private Google account and general Google properties like youtube, and two separate Gsuite accounts), individual Facebook, Reddit and HN containers, a separate containers for various banks I interact with, another separate for online purchases (plus goodreads, because I don't want Amazon to leak), and an individual Pandora one (because why not?).
1: Well, "solved" in that it's harder for them. They can still track me, but at least now they get conflicting cookies from different types of sites but the same IP which might confuse their metrics some. I'm aware I'm probably just making it harder to state anything about me with too much confidence at most.
Highly recommended tho.
Even if there is no malice, it seems that they are not enforcing strict boundaries between their products.
It's still a Google service tied to your account, gsuite or not. It doesn't surprise me one bit that there's still a bit of integration there. They already have your email and the emails of others in your address book that you willingly gave them. It's not like they're mining you to steal those and give you creepy suggestions.
I could be, and am open to being, wrong, though. I'm not Google. Just found myself with substantially less targeted advertising once I left.
And actually that's slightly interesting - how does one determine a bug re: being the recipient of targeted ads when you shouldn't be? Probably wording it wrong without coffee in me, but I hadn't thought of it like that.