Every 4 years, there's a halving, and there has been endless speculation about this design decision by Satoshi. Is it to coincide with presidential election years in the US? Or the Olympics? Why 4 years? I'm not going to speculate on geo-political reasons here, but I will show you why there's a 4-year cycle and how it's related to the 21 million Bitcoin limit.
If you've watched rent, there's a famous song from it called 525,600 Minutes. That's how many minutes are in a 365-day year. And if you multiply that by 4, you get 2,102,400 minutes. Of course, there's a leap day to consider and that gives us another 60 * 24 = 1440 minutes for a grand total of 2,103840 minutes.
What's remarkable about this number is how close it is to 2.1M. And indeed, if every block is 10 minutes, 2.1M minutes turns out to be 210,000 blocks. And that's the real number for the halving cycle, not a specific 4 years. And indeed, 4 years is slightly longer, by 3840 minutes or 64 hours or 2 2/3 days. The 4 year cycle is really a ~3.96 year cycle, strictly speaking because of this extra overhang.
Block 840,000 will start the 5th epoch of Bitcoin (looking to be around April 20, 2024), which will have 3.125 BTC as a block reward. And that block reward halving every 210,000 blocks is how we get a 21M limit.
The first epoch's block reward was 50, the next 25, the next 12.5 and so on. If you add these block rewards up, that's 100 * 210,000 or 21M. But since we represent these with 0's and 1's and don't have infinite precision, the actual complete amount is slightly less at 20,999,999.9769 BTC.