We know that 21 million Bitcoin cap is fixed rule in code that can never be changed.
When it's only code, it can be changed. It's only if Bitcoiners would accept such a change and I doubt that so far.
New bitcoins are created by miners and amount they get is cut in half every four years in halving.
The block subsidy (excluding transaction fees) is "cut in half" every 210,000 blocks which happens to be almost 4 years when blocks are generated at an average temporal distance of 10 minutes (it's actually a binary bit-shift because you can't cut an odd integer number of satoshis for block subsidy exactly in half).
And by the way the total number of emitted bitcoins is slightly less than 21 million, but that's pedantic nitpicking. But if someone wants to be accurate, it's at most 20,999,999.9769BTC and some coins have been deliberately burned out of existance, e.g. when a block miner didn't collect the full sum of block subsidy and transaction fees (this happened very rarely in the past). Additionally the 50BTC of the Genesis block are unspendable (new code to cover this edge case could make them spendable, but likely nobody wants that and Satoshi Nakamoto very likely won't spend them anyway).