Return=volver a more formal word. Get back a less formal equivalent. Come back=volver aquí Go back=volver alli. They show direction.

Return/go back/get back + verb in infinite. Ex: I get back to work now. Come back has different usage. You can refer to the Cambridge dictionary to know more about it.

I hope my answers were coherent. I hate this app. It is terrible for messaging people.

I need to return/get back/go back/come back watching the documental I need to return/get back/go back/come back to watch the documental. The sentences would works the same way as the first two sentences. Note that the expression is get back to watching or to studying. The word is documentary. So to recap, the nuance between the use of study and studying or watching and watch is somewhat the period of the interruption. And there can also be a difference between I will return to watch the documentary and I will get back to watching the documentary. the first sentence means that you are going home so you can watch the documentary. The second one means that you were watching it but we're interrupted in it and are now going to get back to watching it

I will return/get back/go back/come back studying chemistry. You have been studying chemistry in your room but got hungry. You ran down to the esquina to buy an arepa. You see a friend and have a chat. You say I've got to go and he asked where. You answer: I will return TO studying chemistry. (Awfully formal. You probably would not say that to a friend. But your dad may tell you return to studying chemistry.) I will get back/go back TO studying chemistry. I am not sure what situation would cause you to say. I will come back to studying chemistry.