It all comes down to the confession really. With the very thin physical evidence they had, the jury should not have convicted SA. But they all knew Dassey confessed, so they connected the dots to make it work.
Without the confession Dassey would not have been convicted either. Getting the confession through coercion and then basically reading a dramatization of it at a fucking press conference before the trials should have gotten that guy disbarred.
All the rest of the police planting evidence, yea maybe... police corruption... almost certainly... police pinning everything on SA regardless of the truth... absolutely.
But it all starts and ends with that confession, without it none of this happens.
DO NOT TALK TO THE FUCKING POLICE WITHOUT A (good) LAWYER.