This is the most reasonable explanation at the moment.
https://twitter.com/ArcaChemist/status/1167543733245472768Sasha Fleyshman(C)
"This is offset by what is called a "Difficulty Adjustment", where the 'hash'(random generated number) that is required to solve the block is adjusted - this occurs every 2016 blocks (14 days). So, for all related purposes, 6 blocks/hour is a safe metric to use.
By multiplying that number to expand it to blocks/year, then blocks/cycle (a block cycle is the time between block halvings, where the reward for mining a block is cut in half). That is where the ~210,000 number comes from.
Next, they ran a summation notation of the block rewards approaching zero, as shown by the second image in the above graphic. That number comes out to be 100.
By multiplying 210,000 by 100, you arrive at 21,000,000 BTC - so that hard-capped supply number is just a math based output of the amount of blocks mined multiplied by the rewards for mining each block over time as rewards approach zero.
Now, pulling on this string opens up a new line of questioning, of which I have no answer (as of yet); just theories.
I think it is very important to question what you do not understand with #Bitcoin - the answer is not as important as the thought process."