Modem/router setup for Orbi

That depends on how you are going to use Orbi, as the main router or in the AP mode.  If you want your D7800 to handle your DHCP and be your main controling router you connect Orbi and put it in the AP mode in the interface. If you want your D7800 to act as a gateway modem only and have Orbi act as DHCP server and use full Orbi controls, you bridge the router part in you modem / router (in your interface with the D7800) and set Orbi in router mode.

Setting Orbi up in AP mode is simple.

Advanced>Advanced Setup>Router / AP Mode

...or

Bridging your Netgear modem router is here:

https://kb.netgear.com/24105/What-is-bridge-mode-and-how-do-I-set-it-up-on-my-Nighthawk-router

Which is better for you depends on your individual need and what else you have going for you in your circumstances and environment. Your existing router device is AC2600, your Orbi is AC3000, slightly better bandwidth.