I don't know if this has been covered already in this thread but there is only one Pie firmware release for all the carriers. This means that if you have an unlocked Note 9, you can insert an AT&T sim and the phone will boot into the AT&T firmware with the apps that AT&T wants you to have including DIrecTV and all that bloat.
If you insert a T-Mobile sim, the phone will boot into the T-Mobile firmware and so on, just like what's been happening with the Note 8/S8 since Oreo.
I'm downloading the Pie update from XDA and I'm just going to jump in without flashing the AT&T firmware Oreo firmware onto my phone and then updating from there because there's no point, it's just wasted steps.
The best thing about all of this is that each carriers features will work such as VoLTE and WiFi Calling.
I heard that Pie would do this I'm excited for this!