The Bitcoin network itself pays the miners in the form of a block reward of 12,5 BTC plus transaction fees in each block.
Bitcoin miners are basically competing to find the solution to a mathematical puzzle, when this number is found they receive the block reward + fees.
Bitcoin transactions are also processed this way, each time a block had been found, transactions are added to the ledger (blockchain) and miners are rewarded for that.
I hope I've kept this simple enough to understand, all other articles explaining mining were pretty long.
In case you still want to know a bit more:
https://www.coindesk.com/information/how-bitcoin-mining-works/