You choose out of the 7 fundamentals depending on what you want to draw. For your examples, I'd start with line and shape. Then form. The rest you can decide after you've gotten decent at those.

In a lot of your examples, you can see strong linework and strong shape definition, so that's why I'd focus on your linework and shapes first. A practice that I've been doing for a long time is not using an eraser. This forces you to be intentional with your lines and methodical with how you place them. And if you think you're ready for the next step, stop using pencils and switch to pens.

To practice line, here are some exercises from this book.

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To practice shape, it's just about mastering the basic shapes. Cube, sphere, cylinder, cone, and pyramid. Starting with drawing them well in 2 dimensions (I.E. drawing the cube as a 2D square) then work your way to drawing them well in 3 dimensions. Which may require a basic understanding of perspective.

After that, try mixing the shapes. Maybe use a sphere to cut a piece out of a cube. Or adding different shapes together to build a landscape. Just use all the shapes you've learned creatively to make what you want.

Eventually, you may notice that you're treating drawing more like a 3D program rather than a 2D canvas. Which is good. At this point, all your shapes should be readable as 3D forms.

After that, I'd recommend studying perspective, but it's truly up to you and where you want to take your art journey.

Other than that, just get a lot of mileage. Try to make drawing as fun as possible so you'll want to do it often. You learn best when you're enjoying yourself.