My recollection is that the problem stemmed from having a single sensor that detected pitch or wind speed, and that sensor controlled the automatic trim system they installed to compensate for the bigger engines on the 737 Max.
When the sensor failed, the elevator trim started pushing the nose down, and the forces on it became so great that you could not manually correct it.
It should have had a redundant sensor to make certain there wasn't a single point of failure that could cause this issue.
The MCAS system was just supposed to assist pilots on takeoff by compensating for the larger engines installed on the 767 Max. They did not have to have it and they could turn it off.
In one of the crashes, they did turn it off and sort of regained control of the plane. Then they turned it back on, and it screwed the elevator trim further to push the nose down.
It was pretty much a design error, and lack of training people to deal with the new MCAS system.
A lot of people blamed Boeing's HB-1 Visa engineers/programmers from India. I don't know if that was even a factor, but Boeing should have caught this problem before releasing the aircraft to the airlines.
The MCAS system was required because of a fundamental flaw in the aircraft’s design, namely that they had taken a 1960’s design and retrofitted it with larger, more fuel-efficient engines by moving them forward and up, which changed the aerodynamics making the aircraft’s nose rise and therefore vulnerable to stalling.
bttt