Short answers, pulled from the story.
MICROSCOPE was designed to test the universality of free fall, known as the equivalence principle, which holds that two bodies of different composition fall identically in the same gravity field. The satellite could check this principle 100 times more precisely than is possible on Earth.
MICROSCOPE was launched on the 25th of April 2016 at 21:02:13 UTC. Its decommissioning was announced on the 18th of October 2018 after it completed its science objectives, and the final report was published in 2022.
MICROSCOPE was operated by CNES. Its main experiment, the Twin-Space Accelerometer for Gravity Experiment, or T-SAGE, was built by ONERA.
MICROSCOPE used two differential accelerometers in succession. A reference instrument held two platinum-rhodium alloy masses, while the test instrument paired a platinum-rhodium mass with a titanium-aluminium-vanadium alloy mass called TA6V, comparing whether the differing metals fell at the same acceleration.
On the 4th of December 2017, the first results were published, showing the equivalence principle held true and improving prior measurements by an order of magnitude. No violation appeared at the precision the satellite could reach.
After being passivated, MICROSCOPE deployed two 4.5 metre inflatable booms from the Innovative DEorbiting Aerobrake System, or IDEAS, to create a higher drag profile. This is expected to bring the satellite back into Earth's atmosphere within 25 years instead of 73.