The short version is that customer data was obtained from Facebook and then used in support of one sides election campaign without being acknowledged. In the simpler analogue world, all election advertising has to be imprinted with both the name of the printer and the party agent publisher. This was widely covered in the press, here is a list of the Financial Times articles on the subject.
https://www.ft.com/stream/adb58fc4-3248-4b80-96fe-53e0d9a49870
Part 2 - Assess - Timeline - Present - Current Politics