24 hours cause Egyptians split their sun dials and star decans into 12 parts each (probably cause 12 is divisible by 2, 3, 4, and 6) which the greeks later turned into equal length hours (before the length would change over the year)

60 minutes due to Babylonian base-60 math

12 hour format is just tradition at this point, but derived from the sun dials which only worked at day, so half the time, and the star decans which only worked at night, so the other half.

i mean pretty much every country except the US uses the 24h format on digitial clocks now