ELI5 Leap Years - why was 2000 a leap year but 2100 isn’t?

How did ancient civilizations in 45 B.C. with their ancient technology know that the earth orbits the sun in 365 days? What is the exact length of an Earth year? How does the Gregorian calendar adjust for this discrepancy? There are some exceptions to leap years right? What is the Birthday Paradox? And does it apply to leap year birthdays?

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