No crank no start?

After driving all day doing birthday party errands, I came out of the last store and my jeep wouldn’t start. I wasn’t having any issues leading up to the issue. The first three attempts to start the vehicle it cranked started and slowly shut off within about two seconds. The second two attempts the jeep only cranked but would not start. The last few attempts. The vehicle would not crank nor start. I had one mechanic who told me it was definitely the starter only until later told me he wasn’t sure, so I purchased a starter, climbed under my vehicle and replaced the starter myself. My vehicle does not start, still no crank either. My battery voltage is 12.2 and when the mechanic was attempting to start the vehicle somehow, he got it to crank, and we heard a noise that just sounded like the engine was struggling. I don’t know how to explain the sound and there was a small bit of white smoke that came from underneath the vehicle in the front.

The mechanic says that it’s my engine, no machines were attached to the car and that is a $3,000 repair. I don’t know if it’s something that is connected to the engine or if it’s the engine itself but if I can save myself a $3,000 repair, I would love that I just need help figuring out what it could be. When the key is turned, the lights all come on, and the radio works the key, remote unlocks and locks the doors as well as turns on and off the alarm.