Can anyone elaborate on what it means exactly, please?
It just seems like a weird phrasing for saying that the miner has complete control over which transactions are included in the blocks they mine.
Does it really mean that those particular miners who mined the block can do whaterver they want with that block, like setting txs fees and putting txs? What about the other miners who didn't find that particular block? They become powerless until another block is mined?
Miners can select which transactions they wish to include in the block they are mining. However, they cannot change the fee for a given transaction (except in certain uncommon signature types). In transactions using
SIGHASH_ALL, the fee is implicitly signed into the tx since all inputs and outputs are signed, and if a miner attempts to alter the fee, they will invalidate the transaction, and thus the entire block. Pretty much every wallet I'm aware of uses
A miner is powerless if they don't find a block simply because they cannot control what another miner picks for their block.
Note that after a miner mines a block, they can no longer change the contents of that block, as it would invalidate the proof of work.