don't need to
no box spring normally with a platform only thick mattress really just depends on what you want or already have
A friend of mine bought a platform bed and only uses a mattress with a thick foam mattress pad and said it's perfect.
Depending on the height you want but normally only the mattress.
It will depend on what you like.
You can do either. You often use just a mattress, but if your platform can accommodate a box spring (regarding height) you can add one.