Both the present continuous and "going to" can be used to talk about future plans even though they are present forms, because the future plan already exists in the present. The present continuous is often used for fixed arrangements like appointments, while "going to" is normally used for intentions and decisions. Examples of each include talking about studying this weekend using present continuous versus planning to study hard next week using "going to".
2. Both the present continuous and 'going to' can be used to talk about future plans and arrangements even though they are present forms. This is because the future plan has already been made and exists in the present, it is not something which is decided at the moment of speaking.
3. Often there is very little difference between the two forms and you can use either, however we tend to use the present continuous for a fixed arrangements, such as appointments. Examples: I am studying this weekend She is meeting at 12:00 with her friends
4. For intentions and decisions we'd normally use 'going to‘ Examples: We are going to study really hard next week. He is going to watch a movie tomorrow.