Then what is the point in a defense if it isn't their job to prove a crime wasn't committed? Sit there and point out discrepancies?
That's not sarcasm, that's a genuine question.
There are resources that would better explain this to you, but yes that's the idea.
The idea that a person is "innocent until proven guilty" means that the onus is on the prosecutor to prove the guilt of the defendant, and the standard to which they must prove their case is the point "beyond a reasonable doubt". Therefore it follows that the defense need not prove innocence, but just supply a reasonable doubt, which is much easier.
Say you're charged with shooting a man to death. It happened in broad daylight, there were witnesses, and your fingerprints are on the gun. Seems pretty obvious that you're guilty.
Now let me introduce reasonable doubt: your twin brother was standing right next to you when it happened and his fingerprints are on the gun as well.
All your defense needs to show is that your twin also had means, motive, and opportunity and you can possibly get away with this. The defense doesn't need to prove that your brother is guilty (which in turn proves your innocence, since only one of you shot this victim) only that it could have been him. The fact that it could have been him definitely falls under "reasonable doubt" and therefore you must be found not guilty.
That example is very dumbed down, but I hope it illustrates the difference.
Believe what you want, the verdict disagrees with you.
You shouldn't really ever post anything like this, since the verdict doesn't necessarily equate to the truth. Even in the eyes of the law, it doesn't hold much water: you can be acquitted in a criminal trial and found guilty in a civil trial or vice versa, due to different standards of proof.