We're following updates on the iOS 14 Apple Maps thread. Based on the images at WWDC, looks like it will be tied to the EV App in Car Play and limited to certain charger/vehicle types.
My guess is that Apple is getting this data from the manufacturers themselves and relying on them to provide the data from the vehicles to the app via an update to the CarPlay software. Not much different than the ability for manufacturers to integrate their systems with CarPlay to let you know when you are running low on fuel or to control certain car features through CarPlay...again, my guess...