The latest block, a random one, or any height you type. It is fetched live and its proof of work, merkle root and structure are checked right here in your browser. No server. The validator is the page.
Two layers. A block's proof of work, merkle root, witness commitment and structure all live inside the block itself, so they verify with no extra data. Scripts and signatures need the spending history (the prevouts), which isn't in the block — so click any transaction to fetch it and run its scripts and signatures through the engine for real, right here in your browser. That is the full check, one transaction at a time.