Turns on with jump start but then shuts off because the battery drains

So my car suddenly would just shut off on me and I would turn the key off and back on and it would not start and the voltage on the battery would drop. So I put a new alternator thinking that was the problem, but it was not so what could it possibly be? And then had a guy come test it and camshaft position sensor, oxygen sensors one and two would come up, but he says those are not the problems. The cars does turn on with jump start but then shuts off because the battery drains.