Actually Grp AF on the D850 uses 13 AF points, they're just represented by the 4 shown. Grp is a close-focus priority mode and will generally (although, not always), focus on the part of the subject that's under the AF area closest to the camera. It can use any point in the array to do so. It also tends to lock on faster and more reliably than other modes, since each AF point carries the same "weight".
The Dynamic modes have a primary AF point. Ideally, you use that primary AF point and if you stay on target, it'll continue to use that point. However, if your primary AF point slips off target, one of the other points in the array will attempt to take over. However, on the D850 (and D5/6, D500), the camera will only wait a very short time for you to get back on target before it tries to focus using the primary AF point again (this delay is set with custom setting A3). So, it's like single point AF with a safety net.