[Question] - Role of full nodes

Alright so, if I understood it correctly I can run my very own full node e.g. by running the Bitcoin Core software on my desktop and broadcast transactions on my own. Therefore I do not trust a random node from my hardware wallet provider - so far so good.

I have read by running a specific version of the software you have a vote on potentially changing the network - if enough nodes agree on this specific change the rules have changed for the entire network, right?

What if an external entity, let‘s say a government or a huge company, boots up 40 thousand nodes and is able to have majority on changing the rules of the network? I‘m a bit confused about the role of nodes and having impact on the network. If someone could elaborate I‘d really appreciate that!

Thanks :-)