Hi @digital I removed some parts of your post because they mention a YouTuber who is known to spread misinformation. I don't want him to get any traffic from this forum, so I've removed the link, and mentions of his insecure phone and OS.
One problem with microG is that it actually uses proprietary Google "blobs," so it's not as "deGoogled" as some people might expect.
I'd suggest reading two sections on the website: one on hardware identifiers and the following one on non-hardware identifiers. After reading those two sections, you should have an idea of what regular apps can access, including Google apps.
digital Can Google associate the phone number I use in WhatsApp
No, not unless WhatsApp shares it on purpose or if they use some Google service and accidentally leak it via crash logs or something. But considering how big Meta is, I'd imagine they use their own internal stuff rather than rely too much on Google libraries for such things (but this is just a guess, I don't actually know).
digital Can it associate the account I use to login in my bank app to the Google ID?
Again, depends on how the app works. If the app doesn't share that info with Google, or if you don't use Google for single sign on somehow, then I can't think of a way for the info to be leaked to Google.