Hello guys! Hope all is well.
I have a question about theory or song analysis on this song called Highway 1009 by Enhypen. I'll put the link below.

I initially I thought this is key in C. and almost every 4th bar they put D Major (root F#). But some people say the key is G major. Well I honestly am sure it's C but I can't convince others. How should I approach this? Or this song can't (or shouldn't?) defined key? Some people suggest it's mode because it shows C major scale but also F#?
It'll be great if someone could answer this. Thank you!!

here's the chord progression. Correct me if I'm wrong.
C-Em/B-Am-Em-F-Em-D-G