I tried an experiment: leaving OTG disconnected, I turned the RC on, turned phone on and disabled Airplane mode. I then long-tapped the WiFi icon to see a list of possible connections and chose the "Spark-RC-68f5f0". Under that selection says, "internet may not be available". I then turned Spark on and waited for the RC green light to indicate the drone and RC were linked. I returned to my phone's main page and opened the DJI Go 4 app and saw a little green dot in the lower left corner of the screen that says "Connected" and a "Go Fly" message in the lower right part of the screen ( had 9 satellites indicated). I tapped "Go Fly" to get to my flying screen and saw I have a WiFi 5.8G connection displayed. Camera is working and at this point, I would be able to fly using the cell phone, but I'm inside and don't want to fly.
I then turned the Spark off, exited Go 4, turned off WiFi on my phone, enabled Airplane mode, turned Spark on, waited for the green light to establish link from RC to Spark. I then connected the OTG cable from RC to phone and got a screen message saying, "Open DJI GO 4 when this USB accessory is connected?" and I can choose "OK or Cancel". There is also a box (circle) I could check to "Use by default for this USB accessory" if I wanted to automatically open the Go 4 app by simply plugging-in the OTG cable. When that message pops us, you have to make an "OK" or "Cancel" choice. When I go to the flying screen, I see that I have a WiFi 5.8G signal. So, to answer your question, yes WiFi is visible whether using OTG cable or not, which is how the RC and drone communicate.
