Will I be able to use cellular data when i'm connected to another device via WiFi Direct? If a phone is connected to another device through WiFi direct, will that connection prevent the phone from using cellular data?

Edit:

An iPhone is connected to a dash cam using WiFi Direct (the dash cam shows as a WiFi network when turned on and the iPhone connects with the dash cam by joining the network and putting in the password).

I've read somewhere that iOS can't connect to both WiFi and cellular data at the same time. I just wanted to know if using WiFi directs prevents an iOS device from utilizing cellular data.
From experience and I hope design, Wi-Fi is prioritised above cellular when it comes to connection. Only Wi-Fi assist can be used to use cellular despite Wi-Fi. Also, battery discharge rate drops when on Wi-Fi even when cellular is ON, I believe it is not being used. I cannot provide references.  

Now since dash cam is using Wi-Fi to stream video, not for internet, apps cannot detect the purpose. They'll treat it as logged-out network, or simply a disconnected from internet network.