When someone says tum its out of less respect OR cause the person is younger than them. And when sayin aap the person is older and or they deserve a higher level of respect!
So zoya called aayan tum but she calls asad AAP.
Aayan calls asad and zoya as aap!
Asad calls zoya aap and ayan tum!
Dissect this and we see that aayan is referring to two people who are older than him and symbolically bhai bhabhi types as Aap.
Other than that asya only give eachother that higher self respect!
Asad also calls tanu msk as TUM.
AsYa relationship=sacred, of another standard, mature, respectful.