The trend for top FAs these days is to go to a winner--the team that can best get them a ring. I don't think it has much to do with anything else. Taxes, etc. They do have to be convinced they have a starting spot on the team, but I don't think money is a concern. The pay is about the same everywhere--give or take. The big concern is the ring. So it was with Durant.
As far as Hayward is concerned, I think the Celtics have a leg up on everybody else. They are clearly a Hayward away from seriously contending the Cavs--though they need more beef up front as well. And Stevens was the kid's college coach. Plus the East is the weaker Division--an easier route to the finals.
You figure GS is pretty much starred up, SA maybe but I doubt they can add any star without weakening somewhere else, and Utah, which is in a tough position vs the elite in the West.
Miami? That sounds like fake news to me.