How did physicists discover the speed of light originally? Also how did they discover absolute zero

like any other idea attempts were made to measure it

the idea that sound had some speed was known for a long time Newton a others made direct measrrements

Light was just "too fast" the first I know of was by timing the Jupiter moons. due to the orbits of Jupiter and earth the distance varied by a large amount but the moons were known to go around at regular periods. by comparing the predicted time to see them come around jupiter with the actual time, the speed of light could be measured

Maxwell made a prediction for the speed of electromagnetic wave base ONLY on the electical properties of material and "empty space" that predicted speed was so close to the measured speed physics knew they were ( almost ) sure about it

Later an attempt by michaelson and Morley in 1890s to measure the speed of the earth and light going in different directions FAILED. it keep getting the same speed for light when it was certain the C and earth V should add, like the speed of a bullet form a moving airplane

Einstein said "what would the universe be like if C was the same for all possible observers . solving that question which took many years lead to his Special Relativity

Absolute zero was "measured as the end point of a graph of temperature verses energy.
it was never reached BUT modern experiments have come with 1/000 or more of 1 degree of zero absolute. it is advanced physics to explain

I actually measured C with "table top" spinning mirrors as a physics experiment. it is not too hard. light beam hit a spinning mirror, bounced to a second mirror and back and hit the side of the sinning m which had moved slightly in the time of travel ( a few tens of microseconds)
it was just enough to deflect the returning beam as seen through a small telescope
I impressed even myself, ( and that is not easy)

The first real estimate for the speed of light that I am aware of was the result of formulations derived by James C. Maxwell for his paper on the Dynamic Theory of the Electromagnetic Field (ca. 1865). This value, and the fact that it was necessarily treated as a constant, inspired Dr Einstein to do his subsequent efforts in the area of Relativity.