So, the techniques you're learning are there to develop your sense of 3D space, that much you've got. If you practice those techniques for long enough, you can start to take shortcuts and use fewer and fewer of those construction helpers. For example, I practiced animals for quite a while to the point where I didn't need to draw out a full box at all for the muzzle, I could kind of separate things into planes without drawing it explicitly. Similarly, with that kind of grasp on 3D space, you can start drawing things from imagination - but all good made up things are still based in reality somewhere down the line and therefore you'd need reference to draw them because we're not born with a visual library full of stuff to draw on.

If you watch some of Uncomfortable's Twitch streams you'll see that even for his comics, he still uses a certain amount of construction when he's not 100% sure how something will sit in space.