Actually, the Mueller investigation was an investigation into Russian interference in our election. It concluded that they had done so in "sweeping and systematic" fashion.
Mueller did not conclude that the Trump team's behavior vis-a-vis that effort rose to the level of criminal conspiracy. He did suggest that often enough, obstruction had kept him from the evidence though.
Mueller did not address collusion, as there is no such crime.
He strongly suggested that obstruction had occurred, but that he could not charge a sitting president with a crime, that Congress was empowered to address that.
Robert Mueller did not address Trump's finances, his buck raking, his nepotism, his shocking security breaches or any of the myriad lesser crimes that Trump commits on a daily basis. But Congress is in the process of conducting oversight. The Trump administration is asserting that they have no such power. The courts seem to be favoring the law though.