When Bitcoin first launched in 2009, there was no premine. The Genesis Block, which is the first block in the Bitcoin blockchain, was mined on January 3rd, 2009 by Satoshi Nakamoto.
The block contained a reward of 50 BTC, which was the only way to generate new bitcoins at the time.
However, after the launch of Bitcoin, some people began to realize that it would be possible to premine a cryptocurrency. This would involve creating a new cryptocurrency with its own blockchain, and then mining all or most of the coins before releasing the currency to the public.
While this would give the creators of the currency a large amount of coins, it would also create a centralization of power and wealth.
Fortunately, Bitcoin was not premined. Satoshi Nakamoto released the Bitcoin software to the public without holding any coins for himself.
This allowed anyone with a computer to become a miner and earn rewards for verifying transactions. As more people began to mine Bitcoin and generate new bitcoins, the distribution of wealth became more even.
While there are some cryptocurrencies that were premined (Ethereum, for example), Bitcoin was not one of them. This ensured that anyone could get involved with mining from the beginning, and that there was no centralization of power or wealth within the system.