Google has confirmed that Android Auto Wireless will also work on Android smartphones running Android 8.0 Oreo. In its initial press release, it stated that only devices running on Android P would support wireless Android Auto. However, Google has since changed that. Meaning that most current smartphones will work with Android Auto Wireless. However, you still need a head unit or a vehicle that supports Android Auto Wireless.
Android Auto Wireless essentially needs two things to work. One is that the head unit or the vehicle needs to have a wireless connection (currently there are two head units that support it), and you also need a smartphone that can support Android Auto Wireless, which is going to be Android Oreo or later. However, it appears that users will need to get an update for their phone for that to work. A bit unfortunate, but that's what is needed to get Android Auto Wireless to work. The reason it needs a WiFi connection to work, is because Bluetooth is not strong enough to be able to push enough data to the larger display on the vehicle (these are usually 7-inch or larger displays). That is the main reason why Android Auto Wireless is so fragmented at this time.
Google did show off Android Auto Wireless at I/O earlier this month. It is a pretty simple thing here. Essentially you should be able to get in your car, turn it on and it'll automatically start up Android Auto. Since it'll automatically connect to WiFi in the vehicle. It's similar to automatically connecting to Bluetooth. Which is going to make it a much better user experience, but it's definitely going to take some time before it is available on all Android Auto vehicles. Especially since the car industry can be very slow moving at times, in fact it has taken several years to get Android Auto into around half of the vehicles being sold right now.