Based on the current difficulty level of the network, the miner adds a “nonce”, basically an incremental counter for each guess they take, and hashes data from the prior block and the pending block transactions which yields a fixed size big number (total input could be one character or LOTS, that doesn’t matter). If that hash output number has enough leading zeros to satisfy the network difficulty level, they get to mine the block. Since hashing yields completely unpredictable outputs for every input, you just have to keep changing the nonce and see what happens.

The beauty is that once it works, is very simple to confirm you earned the right to mine the block with that nonce (guess).

Including the prior block’s hash in that calculation, links it to the new block so nothing can be altered in the past, because the hash would no longer verify if ANYTHING changed. Immutability is ensured and easy (ish) to verify.

The difficulty is adjusted every 2,016 blocks so that you need a hash result with more leading zeros if the average block finding time is less that 10 minutes and less leading zeros if the blocks are discovered in more that 10 minutes.

I’m pretty sure this is the idea HA…