Ok, from the beginning.

Heat are desperate to trade for Giannis, they cannot wait till summer, because other teams might have better offers (Lakers or someone with a high lottery pick). They would probably offer Herro first, but I don't think Milwaukee would be interested, you have to extend the guy, and he's expecting a lot of money. They don't have many valuable picks to offer so they would have to look elsewhere. Here comes the Spurs.

San Antonio would send unprotected 1st from Atlanta in 2027 and their own in 2031 with a possibility to swap for the pick from Sacramento (unprotected). They also have 2 players on expiring contract (Barnes and Olynyk) plus a young forward that needs a new team (Sochan as a RFA) - that's a great reset button for the Bucks.

I'm pretty sure Miami would have a hard time trading Bam away, but this could be their only option to get Giannis. In the end of the day they would be swapping Bam, Rozier's contract and 2029 1st for Giannis and Portis.

Milwaukee might chose this option because those picks from Atlanta and Sacramento might have a better value than picks from Golden State. They wouldn't have to take Butler's contract either.

And finally for the Spurs, pairing Wemby and Bam together might take them to a next 'contender' level.