Recently i've come across sentences that doesn't have one in it and it looks like odd to me because i'm used to say which one? the sentences must be correct because they are from … I want to know what the constraints are on using the phrase one of the. Is it used correctly in this example? He is one of the soldiers who fight for their country. Which one is grammatically correct or better?

I have two assignments, one of them is done. I have two assignments, one of which is done. I watched a video tutorial that the teacher said …