Mueller's main investigative conclusions were:
1. The Russians interfered in the 2016 Presidential Election Process.
2. There was no collusion between Donald Trump and the Russians. There was no collusion with the Russians by anyone associated with President Trump or his campaign.
3. Mueller's team also investigated for evidence of obstruction of justice. They covered all areas relevant to the charge.
4. Despite the obstruction portion of their investigation, they did not conclude guilt or innocence. That was because Mueller's team were bound by the long-standing DOJ Office of Legal Counsel (OLC) policy, formulated 45 plus years ago and reaffirmed since. That policy is that criminal charges by the DOJ against a sitting president are not constitutional. (In the opinion of the OLC.) SCOTUS has never addressed the issue directly as it applies to a sitting president.
5. Mueller's report, as well as all evidence about obstruction or any other known criminal issue, remain intact. That evidence may be used by other entities such as Congress to take action against the president regarding alleged crimes. The foremost step would be impeachment. Congress is independently free to act as they deem appropriate.
Additionally, each private citizen can decide for themselves via the Mueller Report as to what guilt or crime in their opinion, (informed or not), is attached to President Trump.
Also, theoretically, it may be possible to indict President Trump after he leaves office.
It will be astonishing if Mueller's testimony varies from the above.
The above tells the whole relevant story.