1) Can you give me in detail explaination of what ma fi dhimma is, and how I can use it to pray my missing salats?

2) Can I say ma fi dhimma before my nafl prayers, and make the intention of making up missed prayers, and it will make up my missed fajr prayers?

3) I usually only read surah hamd in my nafl prayers, not like my farz where its obligatory to recite surah hamd and then another surah like surah Ikhals for one rakat. Do I have to do that for my nafls if my intention is to make up missed farz prayers

1) It means “whatever is my obligation”

So for example when you pray a 2 rakat prayer ma fi dhimma, it will count as fajr if you have any qadha.

2) Yes

3) You must read a complete Surah after Fatiha