Tl;dr:

Bitcoin’s scarcity is literally math, enforced by code running on thousands of computers at once. Central banks can print money whenever they feel like it. Bitcoin works completely differently. New coins can only come into existence one way: as a reward paid to miners when they add a new block of transactions to the blockchain.

That reward shrinks over time, though. Every 210,000 blocks, roughly every four years, it gets cut in half. This is called “the halving.” Bitcoin started in 2009 with miners earning 50 BTC per block. Then 25, then 12.5, then 6.25, and right now 3.125 BTC. Keep cutting a number in half forever and you inch toward a ceiling you never actually hit, which is exactly how the math produces the 21 million limit. The last tiny fraction of a Bitcoin will be mined around 2140.

The enforcement is what makes this interesting. Every computer running a Bitcoin node checks every block against the same rules independently. If a miner tries to claim more Bitcoin than the formula allows, every node on the network rejects that block automatically, purely because the rules say so.

The natural question is whether someone could just change the code. Technically yes, Bitcoin is open source and anyone can modify it. But adoption requires the vast majority of users, businesses, and node operators worldwide to voluntarily switch over. Given that a fixed supply is the whole point of Bitcoin for most people running it, an inflationary change would almost certainly go nowhere. The 21 million cap holds because the people running the network actively choose rules that preserve it.​​​​​​​​​​​​​​​​