I am using an ArduSimple handheld surveying kit attached to an Android phone with GNSS Master software doing location spoofing. In this configuration, QField thinks that it's just using the internal phone locator. We are a bit remote, so I put up our own RTK base station (fixed) central to our reserves. I have used the ArduSimple device with my iPhone, but that's a different configuration where I reflashed the ArduSimple unit to provide WiFi support to the iPhone hotspot. Both configurations work extremely well and give us accuracy down to a few centimetres (I'm not quite ready to believe the 1 cm that it claims.)

QField can connect to a GNSS receiver that outputs NMEA over a network stream if the receiver exposes NMEA via TCP/UDP. If the receiver is doing RTK internally, then the NMEA positions QField receives will already be RTK-refined—so QField should benefit automatically. You can verify it by checking the NMEA GGA fix quality (e.g., 4=RTK fixed, 5=RTK float). The main caveat is that not every WiFi RTK unit actually streams NMEA over WiFi; some only use WiFi for NTRIP/config and output NMEA over Bluetooth/serial instead.