It cranks but will not start?

Hello, I have the car listed above Ex coupe V6. It cranks but won’t start. I checked with machine but no one figures what’s going on and ended up changing lots of parts on it. However, sometimes it does start when it does it’s shakes badly and lots of smoke comes from the tail pipe. So, one of the guys told me head gasket so changed that someone told me coil pack so change that one too and spark plugs. Someone told me some immobilizer system fixed that too even change the coolant temperature sensor too. So don’t know what’s going on any suggestions would be highly appreciated. Thanks