Can you use Wi-Fi while airplane mode is on?

Yes. Supported iPhones and Android phones can use Wi-Fi while airplane mode remains enabled. Airplane mode does not make a Wi-Fi connection impossible.

Apple explicitly allows Wi-Fi use in airplane mode when the airline permits it. Google's Pixel documentation explains how to turn Wi-Fi back on and notes that the phone can remember the previous wireless setting.

Wi-Fi may switch off when airplane mode is first enabled, so it may need to be reactivated separately. Exact behaviour depends on the device, software and saved settings. During a flight, follow the airline's instructions.