You are me a couple of years ago. There are a lot of good info from the answers above but there are a couple of things that you need to understand:
1- The concept of a controller is different from what we were used to. Everything today is done in software. The only hardware you need is an iPad. You phone by the way won’t cut it (too small).
2- The iPad will control 90% of your hardware, including servers, DAC, pre-amp, AVR etc. You just need to download the proper apps from the App Store.
3- The apps don’t have to be matched to the hardware necessarily. For instance I control my server and my DAC with the LUMIN app but neither one of them is from LUMIN. You can do the same with Roon.
4- Lastly, all of your hardware have to be connected to the same network, most likely your wi-fi home network. If your equipment is not connected, all bets are off.