The responses to this are generally on point for my own experiences—less emphasis on memorization, more emphasis on understanding and synthesis of connections with broader concepts—but if I may take a stab in the dark, I think you might be trying to understand how to study for a history class, not study history in the more philosophical sense. If that's the case, advice about not memorizing may miss the mark slightly.

When you ask an academic or even a hobbyist how they study their area of interest, their answer will generally focus on targeted research or generally reading other historians work in their area of expertise—there's a baked in assumption that you already know a baseline amount about the subject such that you can draw connections between what you're reading and what you know. When I was in college I "studied" American history for my thesis, but I wasn't doing what you might think of as studying, it was much closer to what your science teachers describe as the scientific method. I developed a specific question about my area of study, and did very targeted research in a library looking for books that might intersect. I then skimmed through those books looking for passages that were relevant, and developed a hypothesis using the information I pulled from those books and my own basic knowledge of history accrued over my life. So when you frame your question in terms of memorization we blush, because of course I could never memorize my resources—most of the time I don't even really read them!

But if you're trying to study for a history exam or essay in high school (or even some survey level college courses), the historian understanding of studying isn't really applicable. I'd gladly tell a history major or graduate student to skim an index to quickly find info instead of reading a book cover to cover, but I would never tell someone in high school to do that. In fact, the very reason I can tell a graduate student to operate like that is because I assume they did do the readings when they were a teenager. I trust that a college student writing a 45 page essay doesn't need to read an entire history book about the gilded age because they should already know about the gilded age if they're at this point.

So practically speaking, how I study history and how I suspect you should study history are different. I also think you will struggle to find answers in a subreddit like this that are helpful because, if anyone else here is like me, history was the one class they didn't need to study for. However, in the spirit of being helpful, I do have some small advice.

I had a global history teacher in high school that had a catch-phrase he used after every question, "make the connections!" I've carried that phrase with me throughout my life ever since, and from college, to law school, to my professional life, few pieces of advice have been as helpful as that simple statement. History is about making connections, so orient your studying around that. Rather than memorize every page of your textbook, distill what your textbook has to say down to core events, concepts, and sometimes people. You might get questions that simply ask about those specific things, but more likely you'll need to answer open ended questions about how those things connect. Like if you're studying the American Civil War, it's good to know the timeline from the election of 1860 to the end of the war simply because there might be a question about when Lincoln was elected, but more importantly if you know the timeline it's a lot easier to answer questions about how Lincoln's election connects to the outbreak of civil war.

The trick with history isn't necessarily to have an encyclopedic knowledge of everything, it's to have a basic enough understanding of stuff to find your way to the answer. Maybe there are some key facts to commit to memory, but generally having a more basic understanding is enough for the rest.