What is "FQC" test app in Nokia 6? There are some dialer codes in Nokia 6, e.g: *#*#4636#*#*,*#*#2486#*#*, *#*#372733#*#*. The last one starts an application named "FQC". In this app, there is a menu containing "Receiver test", "LED test", etc.

I think this is for factory testing, but after starting this test mode, did my device change to a test device? If yes, how can I stop and exit this test mode? Do I need to factory reset?
Just quit the app and it's ok.

If you change some settings (e.g in *#*#4636#*#*, setting priority to "Only GSM" or "WCDMA"), this will be set on your device. But if you only run it without changing the settings, quitting is enough.

To get your phone on "running test mode" you have to do it on "fbootloader" screen and set it to "factory mode", but restarting the device will get out of that.