The coin developer determines the size of the reward in blocks, the total number of coins, how and when should the coins be distributed?
If you can design a coin, you can set your own rules. You can encode most of the coins in 1 block and mine it all yourself before your blockchain goes live.
Premining is also known as private mining or self-mining, which is a way to accumulate large amounts of coins. This process is carried out by developers within the company in a short period of time with a low level of complexity.
That is, they can mine the entire amount of coins in the genesis block and mine it to their wallet, or can they mine a certain amount of coins up to a certain block height?