When someone says "Bitcoin has over 15,000 nodes worldwide," they mean 15,000+ independent computers are each running Bitcoin software and each maintaining their own full copy of the blockchain. No server owns the truth. Every node checks it for itself.
That single fact — every node independently verifies every transaction and block against protocol rules — is the reason blockchains don't need a central authority. If one node tries to cheat, the rest simply ignore it. There's no admin account to compromise because there's no admin.
Not All Nodes Do the Same Job
Full Node
Downloads and stores the entire blockchain, every block since genesis, and independently validates everything against consensus rules.
