I think this is the best answer. Power may be political power, influence, or money. Regardless, people use that power to change our lives and remove the control we have over our own decisions.
Whoever has the ability to decide the laws and enforce the rules under the threat of legal and physical consequences, and has the mandate and legal protections to achieve it.
Don’t get me wrong, I’m not saying that all laws are bad.
I think “The Establishment” is less of a thing and more of a concept.
It’s a catch-all term for the status quo. How things are and have been for a very long time. When you’re fighting to change things, you’re fighting “The Establishment”, the traditional order of things. Whether that’s corporate greed or political corruption or unfair labour practices, etc… etc…
It’s also the particular individuals who enforce that order and have the power to change it. ie those who currently occupy the corridors of power. The senior leadership amongst politicians, media, courts, police, military. It has plenty of well-paid lackeys who support it and aspire to join it. And the boundaries will change depending on context.