Are your children immortal because they won't die in your lifetime?

the supply is "fixed" in 2 ways. the lifetime maximum btc and in that the issuance per block is known beforehand and purely programmatic.

Let's take a deep dive into the source and the maths, but first we need to understand the current rules, in the beginning from the first block the reward for each block was 50 BTC, every 210 000 blocks the reward is cut in half. To verify this we can read the source code https://github.com/bitcoin/bitcoin/blob/dcfbf3c2107c3cb9d343ebfa0eee78278dea8d66/src/validation.cpp#L1630

So now we can start to test it

These are the present halving, as you can see the tola amount of bitcoin minted each halving decrease and will do that until it is down to 0 sats. So down the road the last halving will have a reward on 1 sats for every block somewhere around year 2140.

So if you would summarise the amount of new coins for all halvings that number would print 21 000 000.

It's disinflationary. Everyone running a node has a copy of the blockchain. Its distribution schedule is fixed. Every 4 years that distribution amount is cut in half in an event known as the halving. Over 92% of all bitcoin has been mined. Do the math. There isn't a hard cap coded into the software, but the distribution halving brings issuance to effectively zero around 2140, which is about the same as a hard cap.

Some people say "There are already 21 million Bitcoin" its just distributed with every block.

fixed supply = the last bitcoins will be mined in 2140.

If what worries you is that it will not reach maximum supply in you lifetime, you can do the maths. You can think about the amount that has already been supplied compared to the amount that will be supplied in your lifetime.

Please read https://bitcoin.org/bitcoin.pdf

All starts there.