You press send. Who checks what happens next?

Verification happens independently

A full node checks the rules for itself. Here are some of the questions it asks. Is the spending authorized? Has this output already been spent? Does the block have enough proof of work? Is the reward the miner claims for the block allowed?

If a block fails those checks, nodes that enforce the rules reject it. Owning more machines does not let a miner forge another person’s signature. It also does not let a miner invent extra subsidy. The subsidy is the new bitcoin that each block is allowed to create.

That is the point of running your own node. You check with your own software, instead of relying only on someone else’s report of what happened. A block explorer is a website that shows blocks and transactions. It is a useful window into the network, but it is still someone else’s window.

Why call it a blockchain?

A block bundles transactions together. To see how blocks link, you need one idea: a hash. A hash is a number calculated from some data. Change the data, even a little, and the hash changes. Each block starts with a header, a short summary of the block. The header includes the hash of the previous block’s header. That links the new block to the history before it.

The links make changes easy to spot. If someone edits an old block, its hash changes and the link to the next block breaks. But links alone are not the whole security story. Proof of work makes replacing confirmed history costly. And a block must still obey the rules. A miner cannot make a too-large reward valid just by working harder.

Follow one payment through the system

A wallet signs a transaction and broadcasts it. That means the wallet sends it out to the network. Nodes that accept it may relay it, or pass it on, to other nodes. Each of those nodes also keeps it in its own waiting area, called a mempool. There is no single waiting room that every node sees the same way.

Mining software picks transactions from the waiting area for a candidate block, a draft of the next block. When a miner finds a qualifying block, it is broadcast and checked. Each new block built on top of it adds a confirmation.

Then why does hashrate matter?

Hashrate is how many attempts per second miners make to find a block. Checking the rules tells you whether a block is valid. Proof of work does two more jobs. It helps the network choose between competing valid histories. It also raises the cost of replacing confirmed history.

An attacker with enough hashrate may try to keep certain payments out of blocks. They may also try a reorganization: replacing recent blocks to reverse a payment they made themselves. That is serious. But it still does not give them somebody else’s keys. And it does not make nodes accept blocks that break the rules.

The network needs more than fast chips

A miner needs up-to-date work to do, called a job. It also needs a working connection to send its results back. A machine working on an outdated job can stay busy without reliably adding to the current chain.

So when you look at the network, see the whole sequence. A wallet authorizes the payment. Nodes relay it. Mining software selects it for a block. Miners do the work. Nodes verify the result. A Miner Technician’s job sits inside that sequence, not outside it.