What are the technical roles of validators & node runners when implementing new core EIPs?

I know the whole ecosystem’s feedbacks are taken in before officially passing core EIPs so they are usually not controversial by the time they get implemented, but technically what are the roles of validators & node runners when it comes to core EIP implementation? My understanding is after clients include the new changes, validators have to choose to update to the new client versions? And if they choose not to update then a hard fork would be created? What about the role of node runners in this process?