The headlights are pretty good from stock. With 3 positions: DLR (peripheral lights), Low beam (middle lights) and high beam (top light). The bottom light is always on. Low beam is switched on automatically when the sensor on the dashboard detects darkness, or manually if DLR is turned off in the menus (this may be useful in fog for example).
There is no obvious spots to put additional lights, and so far not a lot of installs have been published. I’ve seen a very long video with lights installed on the lower tube supporting the front end (but warning the clearance with the forks is very tight) and this very extensive install report on Facebook.
KTM added lights to its catalog: P/N 63514910033, 487€. Those are PIAA lights, made by Valeo for KTM. The hardware simply replace the metallic braces for the headlight, adding mounting points.
The mounting braces are available stand alone as p/n 63514910044, €150 for just the bars without any connector.
Denali offers 790 Adv specific mounts, mounted on the same tube under the instrument cluster, with an additional leg to stabilise it and not just relying on the clamp. $120.
BDCW also offers light mounts and optionally the associated lights to go with it. Plenty of options detailed in this ADVRider thread ($150 for the mounts, $500 for the mount plus some nice LED lights).
Cyclops also offers a mount ($80), in multiple sizes depending on the size of your lights. Also available with your favorite lights and and a bunch of other options.
GIVI also has specific mounts for the 790, reference LS7710. Those are very low, attached to the bash plate, and not compatible with crash guards as they use the same attachment point. €60 or so. See this install report (more pic in the comments).
There is a company in Columbia, Lion, making mounts as well. Their picture is from a S model, it is unclear if a R fender would clear the one fixed under the light housing. $60 each pair. The site is a little scarce on information.
The cheapo option is to buy a 18mm “Stauff clamp” from a hydraulic shop and to use it to mount the lights on the lower bar. $10 for a pair. As seen on FB in a comment by Kris Eric :