Who checks that Bitcoin's rules are followed?

Detailed explanation

Bitcoin has no central register prescribing valid rules to all participants. Instead, full node software contains checks for transactions, blocks, signatures, proof of work and money supply. Each node applies these rules to received data itself. If a block breaks an active rule, that node rejects it.

This does not mean every node has a formal vote or that rules can be won simply by setting up many nodes. What matters is which software users, businesses, wallet providers, exchanges, miners and other economic participants actually run and which bitcoin they treat as valid. These decisions arise through technical review, discussion, coordination and voluntary adoption.

A node cannot force others to use the same software. It does, however, protect its operator from accepting a chain they consider invalid. Consensus is therefore neither a central vote nor pure miner rule, but the combined effect of compatible rules, independent validation and social coordination.