How hard is it to get into grad school and do a PhD

It depends a little on the field, but generally once you are finishing up your undergrad and you have an interest in doing something a little more hardcore in a topic you enjoy, then it's just a matter of applying to some masters programmes and paying the (usually very high) tuition fees. (this is for europe btw). Universities make a lot of money from masters students so they are eager to take on a lot of them, which means as long as you enjoy the topic, have the money to pay for it, and you're prepared to spend 1-2 years of your life basically just studying and nothing else, then it's not too difficult to get into a master's programme and graduate.

A PhD is a different story. Finding a PhD is very much not a meritocracy, you have to network well with researchers at your masters university or somehow have a good correspondence with a researcher somewhere else. They have to LIKE you as a person, because they have to mentor you and put up with you for 3-6 years. For the most part, a PhD student will be doing a lot of the less interesting work related to the research of their supervisor. This is usually the grind-y work that the supervisor needs done but doesn't want to do. It's like an internship but with more prestige. Sometimes you'll get resources to do your own work on the side, sometimes not.

Intelligence is important for this process, but I would say it's more about persistence and sacrifice then just being clever.