This isn’t that complicated. Imagine what would have happened in Iraq or Afghanistan if the president was an intelligence asset they had cultivated for decades. Not to mention tons of senators, represntatives, and influential NGOs like the NRA. Also all those social media psyops that are disappointingly effective at sowing division in the US.
Russia has an incredible amount of influence in the US and is still getting wolloped, although not nearly as much or as quickly as they should/could be.