Your node (or the node you choose to connect to) defines what is bitcoin, according to you.
Everyone gets a vote on what bitcoin is, but the definition of bitcoin that you vote for is that defined by the code running on the node you choose to connect to the network through.
Running your own node let's you speak for yourself. If you connect to someone else's node you're effectively saying "I vote for whatever they're voting for".
The two parties calling toilet roll "bitcoin", are not actually using the bitcoin network. They're using the "toilet roll" network and simply calling it bitcoin.
Their transaction would not appear on my node, or the vast majority of any other bitcoin nodes and therefore it's not bitcoin by consensus.
If you don't run your own node, you're trusting that the node you're connecting to isn't actually running the "toilet roll" network and you didn't just sell your car for a 6 pack of Andrex. With Bitcoin you never have to trust - you are always able to (and encouraged to) verify, and this is what running your own node allows: verification, with no trust required.
The way I understand it: If another person's node connects to your node, your node can stop relaying certain blocks to them. And if everybody stops relaying those blocks to them, then those blocks will not reach them. So in a way, your node can do something a bit similar to "voting" in this situation.
But does a node on a lonely notebook have any real world impact? Do actors who accept Bitcoin payments connect to random nodes?
If so, what prevents someone from running one million nodes on a single laptop and "encircle" such an actor? There is no proof of work involved in running a node, right? So what gives a node any meaning or voting rights?
If someone some how manages to encircle your node with a million of their own (i.e. your node is only connected to theirs) then what are they going to do? If they try to filter blocks or tamper with blocks then your node will detect that and disconnect from them. Ultimately, your node is hard coded with a list of IP addresses, so that if all else fails when trying to obtain good connections with other node (DNS etc) then it can connect to those.
Regarding the impact of a single node on the network, that node is a way that _you_ can vote on what bitcoin is. You could equally shut it down and vote by connecting through someone else's node, but then you'd be trusting that their node defines bitcoin the way you want to.
In other words, the people decide what bitcoin is, not individual nodes, but the nodes that people connect through define what they are voting for. If you want to be _sure_ that your vote isn't being misdirected, that what the node is reporting as bitcoin really is bitcoin then connect to the network through your own node.
If Blockstream wants to change the protocol in a way that Coinbase, Binance, Trezor, Electrum, Metamask, the miners, El Salvador, Grayscale and Microstrategy don't like, they will probably have a hard time pushing it through.
Everyone gets a vote on what bitcoin is, but the definition of bitcoin that you vote for is that defined by the code running on the node you choose to connect to the network through.
Running your own node let's you speak for yourself. If you connect to someone else's node you're effectively saying "I vote for whatever they're voting for".
The two parties calling toilet roll "bitcoin", are not actually using the bitcoin network. They're using the "toilet roll" network and simply calling it bitcoin. Their transaction would not appear on my node, or the vast majority of any other bitcoin nodes and therefore it's not bitcoin by consensus.
If you don't run your own node, you're trusting that the node you're connecting to isn't actually running the "toilet roll" network and you didn't just sell your car for a 6 pack of Andrex. With Bitcoin you never have to trust - you are always able to (and encouraged to) verify, and this is what running your own node allows: verification, with no trust required.