Your English is mostly perfect, but (and as someone who tries, like you, to speak another language correctly):

Could it be that not only these platforms are harmful in mental health and privacy but also physical health?

should be

Could it be that not only are these platforms harmful in mental health and privacy but also physical health?

The change in location of the word are is what makes it a grammatically correct question rather than a statement.

To me, I think the primary harm to our physical health IS the impact on our mental health, which is a physical health. To say nothing of neglecting our physical bodies by being so sedentary.