Complex sentence: There are "büyükşehir belediyesi" (or greater municipalities) for large cities like İstanbul or İzmir and consist of an extra administrative layer run by an elected head mayor, who oversees a number of municipalities and mayors.
Split into simpler sentences: There are "büyükşehir belediyesi" (or greater municipality) for metropolises like İstanbul or İzmir. This consists of an extra administrative layer which is run by an elected head mayor, who oversees a number of municipalities and mayors.