As other have said use
@Steve Z9 setup guide book and his mirrorless editions of secrets to the autofocus system and exposure and metering for Nikon. Also download the Z9 reference guide and the Z9 reference guide supplements for the latest firmware .... they are hand online references.
Use the info in the menu that is there whenever the ? is present you can touch it if your touch screen is turned on or push the ? button on the back of the camera to learn what those menu items impact/do. With your primary lens on mounted going through the menu and reading what it tells you when you hit that ? can be a very fast learning experience and prep for and even more useful time with
@Steve books. I skip the video section since I do not do video.
I came to 2 Z9's from D850 and D6. As others have stated once you get used to how to "play a Z9" the D6 will become surplus unless you need two bodies at once. I am a birder and the Z800mm f/6.3 paired with Z9 had me sell off the last of my DSLR gear D850 and 600 mm f/4 E were the last to go. When I got a second Z9 my wife traded my Z6II in for a Z7II. I have since added a Z6III for the low light viewfinder. I did replace the Z800 with a Z600 f/4 TC, other currnet lenses are a z600 f/6.3, z24-120 f/4, tamron z mount 150-500 and tamron z mount 35-150 f/2-28 (my indoor workhorse)
I shoot manual with auto ISO and use EV frequently, change metering as the situation requires. Things your D850 and D6 cannot do = I also keep an eye on the live histogram in the viewfinder, viewfinders may fib a bit to you but the histogram will not. I set the viewfinder in custom settings page 608 of the Z9 reference guide to show effects of settings no adjust for ease of viewing. That is the "what you see is what you get" setting and usually agrees with my histogram.
I forgot I started with Delkin Black (I had used blacks for years) but had issues with their card readers not want to eject cards from my desktop etc... Delkin sent me a new one that had to be set up just so etc.. I finally got tired of fiddling with the readers. I finally moved to ProGrade Digital Cobalt and their single slot USB 4.0 card reader. Excellent performance in my Z9's and Z6III and they run as cool or cooler than the Delkin Black and handle 20fps and raw files very well.
Good luck and enjoy.