Russian interference in 2016 US elections - This is an investigation of Russians, Trump as far as I know it, isn't a Russian.
links between associates of Donald Trump and Russian officials - D Trump isn't an associate of himself, nor is he a Russian official (as far as I know)
Possible obstruction of justice by Trump and his associates - This is getting close to an investigation into D Trump, but it never happened, as pointed out in the report
Robert Mueller made clear: he couldn't have indicted Trump even if he wanted to | Lawrence Douglas
The only reason Mueller did not seek indictment was because the special counsel’s office “is part of the Department of Justice, and by regulation, was bound by that department policy”. That policy holds that a president “cannot be charged with a federal crime while he is in office”.
Indicting the president was therefore “not an option we could consider”.
Then there is Mueller’s parting statement: “The constitution
requires a process other than the criminal justice system to formally accuse a sitting president of wrongdoing.”
Why the Mueller Investigation Failed
far from authorizing a wide-ranging investigation of the President and his allies,
the Justice Department directed Mueller to limit his probe to individuals who were reasonably suspected of committing crimes. Temperamentally as well as professionally, Mueller was inclined to follow this advice.
Trump’s political adversaries, unaware of Mueller’s determination to run a brisk, narrow investigation, became invested in the expectation that he would uncover such sweeping and devastating proof of criminal misdeeds that a misbegotten Presidency would be forced to come to an end.
But
Trump’s fiercest defenders and
Mueller’s most devoted fans misjudged the special counsel from the beginning.
Mueller
did not use the F.B.I. information as a catalyst for a deeper examination of Trump’s history and personal finances.
Nor did he demand to see Trump’s taxes, or examine the roots of his special affinity for Putin’s Russia. Most important, Mueller
declined to issue a grand-jury subpoena for Trump’s testimony,
and excluded from his report a conclusion that Trump had committed crimes.