Nodes, clients and who enforces the rules
Miners and validators produce blocks. They do not decide what counts as valid. That distinction is the whole answer to who controls a blockchain.
It is widely assumed that miners run Bitcoin, or that validators run Ethereum. They produce blocks. Whether a block is accepted is decided by every node that receives it, independently, against rules the node itself holds.
What a full node does
- Downloads every block and checks every transaction against the rules.
- Verifies every signature, rejects anything that double-spends, and rejects any block that creates more currency than the schedule permits.
- Discards invalid blocks outright, regardless of how much work or stake is behind them.
- Relays what it accepts to its peers.
If a miner with a majority of hashing power produces a block awarding themselves twice the permitted subsidy, every full node drops it. The work is wasted. Majority hashing power allows reordering and exclusion of transactions; it does not allow changing the rules, because the rules live on the machines of everyone verifying.
Why this is the answer to “who controls it”
Developers write software. Miners and validators produce blocks. Exchanges list assets. None of them can impose a rule change on people who do not run the software implementing it.
A change takes effect only when enough of the network runs code containing it. That is a messy, slow, socially-negotiated process with no formal voting mechanism, and it has produced open conflict more than once. See forks and the scaling debate.
Light clients
Most people do not run a full node. Phone wallets typically use a light client, which downloads block headers and asks other nodes for the specific transactions it cares about.
It can confirm that a transaction is included in a chain with a great deal of work behind it, without independently checking that every rule was followed. For everyday amounts that is a sensible compromise. It does mean the user is relying on the honesty of the wider network rather than checking for themselves — which is the ordinary condition of almost every participant.
Node count as a health measure
The number of independent full nodes is sometimes cited as a decentralisation metric. Treat it carefully: nodes are cheap to run and easy to count badly, many are operated by the same few companies, and the figure says nothing about geographic or jurisdictional spread. It is a weak indicator that is reported as a strong one.
Why you might run one
It removes your dependence on anyone else's view of the ledger. You verify your own balance against rules you chose to run, and you are counted among those who decide what a valid block is. For most holders this is unnecessary. For anyone who finds the argument about who controls the network interesting, running a node is the only way to be part of the answer rather than a spectator to it.