The blazing fast and accurate AF of the D5 and D500 is a product of the combination of the EXPEED 5 processor and the Multi-CAM 20K focus sensor. The same combination is used in the D850 as well. It is not a separate focusing processor, but it dedicates more threads in the processor to focusing than older EXPEED versions could do.
The D6 got the EXPEED 6 processor and the Multi-CAM 37K focus sensor, making for even faster AF.
The Single-processor MILCs uses EXPEED 6, but the readout from the AF system to the processor is slower than from the best dedicated modules. The two EXPEED 6 processors in the Z6II and Z7II compensate for this in most situations, but can hunt a bit more in low contrast situations than the best DSLRs
The Z9 sensor has a faster readout than the prosumer sensors, coupled with the EXPEED 7 processor it has the D6 beaten for all types of shooting from what I have read and heard from friends who have got their hands on the new flagship.