Hello. First things first very sorry for the late reply. You probably bought another device or you found a way to use anbox/waydroid. I am still typing because i want to give some information for those who want to install Ubuntu Touch on their Redmi Note 9.
Regarding your question, you don't need adb to install waydroid (anbox is long gone now and waydroid is much better) and you can install whatsapp easily inside waydroid. Just go to OpenStore and download waydroid helper.
For other people who want to learn about this device, it is daily driveable. But there are bugs here and there. These are the list of bugs that i remember:
Connecting the device to a pc doesn't work therefore no sideloading and no adb Brightness slider doesn't work correctly. You can get to the level of brightness that you want but it takes some work. Because brightness slider doesn't work, when you turn off or on your screen for a split second the brightness rises up. This isn't a problem on daylight but may cause small "flashes" on night. Camera notch isn't aligned with the black bar where wifi icons etc. are. Also, this device has rounded corners so you cannot see AM/PM fully.5.This is not a bug but camera software is really bad. But it works. This is the most important bug. Power cycle isn't working correctly. For example, if i try to turn off the phone, it works for 7/10 of the time. In that 3/10 the phone reboots instead of shutting down. Same thing with rebooting. But holding down power button resets the whole phone so rebooting is not a problem.
These are all the bugs. Some of you may find these bugs critical, some of you may not. Also remember: you can reset to stock miui with fastboot flashing. Just hold down volume down and power until you see fastboot and then using XiaomiFlash flash back to stock firmware.