You can put a comforter inside a duvet if you want too and it will be more comfortable as well and make the duvet last longer when you use a comforter inside a duvet.
You can also use a duvet cover to keep the duvet clean and make the duvet last longer.
It is recommended to put a comforter inside a duvet and a lot of people including myself do so.
When using a duvet on your bed you usually only need to use a bottom sheet and then use the duvet on the top sheet.
You can use a top sheet over a duvet if you want too but most people who make their beds with a duvet just use a bottom sheet on the mattress first.
Then they use the duvet on top which is what I do.
My duvet is all I really have other than my bottom bed sheet and that is enough for me.
I've read online before that some people do have a top sheet with the duvet but it's a personal preference.
There's no rules on how you must make your bed and if you want to use a top sheet with your duvet you can but most people do not.