The first guy looks like something from Starwars ^^ love him,Last drawing reminds me a bit of the orc camps you would encounter in warcraft 3 bbq some humans!
I don't know how serious you are about improving, if you draw every now and then as a hobby or if you want to grind, but generally your drawings have good shading and a lot of details, but don't have the underlying structure to hold it all in place, so its like all icing and no cake
I totally get it, its way more fun to shade and detail in the stuff than to draw boxes and cylinders, I totally feel the same way, and have done the same myself
You could check this guy out, he has this overpaint series, where he draws over other peoples drawings to explain what they could improve, and what he as a professional artist would have done differently, which is really fun and interesting to watch.
https://www.youtube.com/watch?v=ITTDUC6POqY
If you want to learn from the very beginning, the super fundamentals in drawing I can recommend this guy
https://www.youtube.com/channel/UCHU...C6iFFQ0EqIoPgQ
which is like the ultimate drawing guide for everything, but it can be very time intensive, so if you are not super interested in drawing you can quickly lose interest.
Here is a good one for drawing heads, which is pretty much the gold standard method.
https://www.youtube.com/watch?v=wAOldLWIDSM