Bitcoins are created by a process called 'mining'.
Computers solve mathematical puzzles to confirm blocks of transactions on a network. When they succeed, they reward you with bitcoin.
The mathematical puzzle increases in difficulty as more and more computers join the network. Therefore, more powerful computers are needed to solve the blocks.
In the early days, you could mine bitcoin with the CPU of a laptop, as the difficultly increased, people started mining with graphics cards.
In 2013, the first ASIC miners were built. These are machines specifically built to mine bitcoin.
Bitcoin is now mined on a industrial-level. There are large warehouses with hundreds of ASIC machines mining bitcoin and other cryptocurrencies now.