Okay. Hm that's weird.
Yes, that's pretty much what I'm saying. Let's say I downloaded an .epub file on my Mac that I want to get into the Apple Books app on my iPhone/iPad to read it there. What I usually do is open the Apple Books app on my Mac, and then just dragging the .epub file from Finder into the Apple Books window (I suppose onto the app icon would work aswell). It's then imported into the app, and if you have iCloud sync for your book library enabled, it will be uploaded and appear on your iPhone/iPad next time you open the Apple Books app there.
Similarly if you want to import an ePub file that you already have on an iPad (for example in the Files app, or in Mail/Messages/...), then you can just start dragging it from there, open the Apple Books app with a second finger (or open it into split view beforehand), and drop the file into the Books app. You might need to have open the "Library" tab (not the "Now Reading" tab or so) in the books app or it may not work. On iPhone there's no drag-and-drop but you can probably just use the share-menu for the same result.
If I later want to remove it, then... well, I just remove it, I'm not sure what you're asking. On Mac, just right-click on the book and select Delete. On iPhone/iPad, press the "..."-icon below the book-icon and select Delete there.
I'm honestly a little confused as to what you need iTunes for. I suppose you can also import books to an iOS device via iTunes, but it seems like an unnecessary obstacle when there's iCloud sync (unless the iOS device belongs to a different Apple ID than the Mac, in which case you could still get the epub file to the iOS device via other means (AirDrop, E-Mail, ...) and import it directly on the device into the Apple Books app).
DRM only protects books you have bought, i.e. books you bought in the Apple Books store might have DRM on them so you can't read them elsewhere without the respective Apple ID. It doesn't stop you from importing your own .epub files into the books app (as long as the .epub files that you are trying to import are DRM-free themselves).