Orbi AP Confusion

Sort of. You can also plug devices into the Orbi router and satellite, but their IP addresses will still be assigned by the Nighthawk, and the Nighthawk will still be the one making the routing decisions. All the wifi will be handled by the Orbi, but both the Nighthawk and Orbi can handle wired connections. Generally, you only want one router on your network (putting the Orbi in AP mode means it won't count as a router for these puposes). You'l get one IP address from your ISP, and then your Nighthawk will give all your devices something in, say, the 192.168.1.x address space, using network address translation (NAT) when it communicates with the internet. But if the Orbi is in router mode, it will will also get an IP address from the Nighthawk, and will have to use a different IP address space, say, 192.168.2.x for devices connected to it. You won't be able to use 192.168.1.x on both the Nighthawk and Orbi. And the Orbi will also have to do NAT to commuicate with what it thinks is the internet, but which is actually only your Nighthawk's network. Double-NAT can cause problems, and is something you should avoid.

And no, the Nighthawk can't function as part of the mesh. Right now, generally, you can only use routers and satellites sold as part of a mesh system (Orbi, Velop, Google Wifi, etc.) in a mesh system. None of them are compatible with each other, and you can't even just add another Netgear router to the Orbi to expand it, can't add another Linksys router to a Velop system, etc. Maybe someday there will be more compatibility, but not yet.

If you use the Orbi and Nighthawk with the Orbi in AP mode, and both with the same SSID and password, you will basically have two different networks with the same name and on the same IP network. But, at a functional level, it won't really be any better than giving them different SSIDs and passwords, other than saving you from having to configure your devices twice. You'll also probably have some problems with some functions. If you use file sharing, devices connected to the Orbi and Nighthawk will be on different networks (despite the SSIDs being the same), and won't be able to see each other. If you use access control lists, any devices that move around will have to be added to both. Same with any reserved IP addresses. And, any changes you make will also require logging into the Orbi to change there, and then logging into the Nighthawk to make the same changes again there. If you use the Nighthawk as the router, and the Orbi as the APs, then you'll basically only have to log onto the Orbi to change wireless settings (SSID, password, which channels you use), and all other things will be done from the Nighthawk (ACLs, reserved IPs, blocked sites and services, scheduling, etc.).