If you have a particular topic you're trying to learn about, there are a lot of ways you can find books and expand your knowledge base:

As far as collections, you have a lot of options. Academic journals publish reviews of books (some journals like American Historical Review are exclusively reviews, but journals with research articles often also have reviews) so you can learn about new books there. H-Net, a forum for humanities scholarship, will also sometimes post book reviews and author interviews. Sometimes you can also refer to "state of the field" essays or historiographical volumes to tell you what's out there. I've also noticed that depending on your field, Twitter can be oddly good for finding new books if you follow many historians?

Sometimes if I know I want to find some recent books, but don't know exactly what I'm looking for, I might also check university press websites. Usually, University presses have a couple areas of focus so if you notice those, they can be really helpful. For example, University of North Carolina Press is generally really good for Civil War stuff, and UPenn is good for early American stuff, especially legal history. So even if you're reading casually and you noticed that all the books you like are being published by the same few presses, it stands to reason that they will continue publishing things in your area of interest. The same goes for journals. Usually they have particular things they are interested in, so if you were to continue looking at what's in the journal, you can find multiple things in your general area of interest.

You can generally also use books and articles to find more books and articles! Notes and bibliography will tell you where an author gets their ideas. With books, you can even have extra fun and see who pops up in the acknowledgments, and you can see what those people are doing which may introduce you to some new ideas.

Having said all that, if someone is formally trained as a historian, reading broadly is part of that education. I graduate level, the expectation is that history students read a lot of books and develop a general sense of what has been written about a field, and for people who work as historians, particularly if they publish, part of that job is keeping up with new scholarship. If you are actively writing research, you will get pretty familiar with some books that you may use over and over again. I don't think this is exclusive to academic historians (I know some reenactors and they are serious as hell about what they do) But just to say that depending on the background of the commenter, cultivating that broad knowledge is a big part of their job, and probably much easier to maintain once they have that baseline from their education.