Imam Mahdi, Isa AS and Muslim army

How will Muslims join Imam Mahdi, where is the Ummah? It has been so fragmented since caliphate. How will we be protected from Dajjal, the Antichrist. It says good Christians and Muslims will join hands. Where are these good Christians. I’m worried about the situation of Ummah. Will Al- Aqsa be destroyed? How will we protect Jerusalem?