Hello @MortgageMan90
In WiFi (in general), client devices make their own decisions about which access point and which band to connect to initially as well as when to switch between them. Google/Nest WiFi (including the new Nest WiFi Pro) support some optional standards (802.11k and 802.11v) that they use to provide clients with extra information to help them make better decisions. This information includes things like telling them which other access points are part of the same network so clients know they can switch without losing any open connections. It also includes signal quality measurements from multiple access points and bands so that clients can tell which other options might be better than what they are currently connected to. The new Nest WiFi Pro should support all three bands for this extra information to help clients that also support 6GHz make good decisions about when to switch between them.
But, these standards are most useful for mobile devices, since fixed devices should just be scanning and selecting a good option when they first start up and then sticking with it. So, most fixed devices don't support these standards, and make initial decisions on their own. Some do a better job of this than others, unfortunately.
Long story short, while the network can provide some enhanced advisory information, decisions are ultimately left to clients.
I will add a caveat that some WiFi networks attempt to be more aggressive about getting clients to switch by literally kicking them off of a band/access-point. This can have unpleasant effects, and may or may not actually help. I don't know whether Google/Nest WiFi does something like this, but I would be surprised if it did. It's mostly something done in enterprise networks that deal with more mobile devices, some of which do not support 802.11k and 802.11v.
I know this was probably a lot more detail than you wanted, but this stuff is a lot more complex under the covers than the simple marketing descriptions imply.
