Validators are chosen to participate in block validation on the Ethereum 2.0 network through a random selection process. The selection process is designed to be fair and unbiased and based on a combination of factors. This includes the amount of ETH a validator has staked, the length of time the validator has been staking, and the overall network participation rate.
Validators with more ETH staked have a higher chance of being selected to validate a block. However, the selection process is also designed to be randomized, which means that even validators with relatively small amounts of staked ETH have a chance to be selected to validate a block.