What is the "certain criteria" to be a valid Bitcoin block?
A valid block is found when any miner authors a block candidate whose block hash interpreted as a number is lower than the target. This is equivalent to meeting or surpassing the required difficulty.

To this end, miners create templates of a block that they would like to confirm by selecting a set of transactions and creating their unique coinbase transaction to collect the block reward. Each block template can be tried a number of times by cycling through all possible values of the arbitrarily picked nonce in the block header. After that, the template can be slightly adapted e.g. by changing the block time in the header, by incrementing the extranonce in the coinbase transaction or introducing any other change into the block template.