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A block has to be proposed, propagated, and accepted. The identity of 'who gets to propose' and 'who decides if it's valid' is the single most politically charged question in blockchain design, because it determines who earns block rewards and who can be bribed or coerced. PoW answers it with hashrate, PoS with stake, and PoA with a whitelist.
On Ethereum right now there are ~1M validators, each staking 32 ETH. Every 12-second slot, one validator is pseudo-randomly selected (via RANDAO) to propose a block. A committee of ~128 other validators attests whether the block looks valid. If >2/3 of the committee attests within a slot, the block is accepted; after ~13 minutes (2 epochs) it becomes finalized — meaning reversing it would require slashing ~$10B of staked ETH. On Bitcoin, the equivalent actors are mining pools — four of them (Foundry USA, AntPool, F2Pool, Binance Pool) currently produce ~75% of all blocks.