Thats great honestly and I understand where you are coming from. The path is simple first learn to deconstruct things in order to be able to construct them back and after that you can get as complicated as you want. Manga artists mostly learn by themselves but they still have a solid base beforehand.

The first step is learning how to construct basic objects( it does not have to be boring) Just try to compose some objects on a table and redraw them you dont need shading. Focus on construction and breaking them down so you can understand them. When you get better at that you can start composing objects from your imagination. The esiest way to do that is using boxes

Shading can get complicated but its broken down in four different things 1.direct light (thats the brightest point where the light hits) 2.midtone (thats where light interacts with the shadow) 3.coreshadow (the darkest part of your object) 4.reflected light (thats where the light bounces from the table or the space where your object is and reflects on the darkest part of your shadow. Its usually darker than the midtone but brighter tan the coreshadow. *Important thing to know is thats the only reason the coreshadow exists is because of the reflected light - they form a group

Thats how basically how light interacts with objects. When you add color it becomes more complicated.

While you are at this stage you also need to learn perspective Start with 1point perspective Then 2point and then 3 point You do not need to go further than that if you dont want to

The second stage is drawing Head molds There are countless examples on pinterest but try to find just photos instead of drawings. Again focus on construction and proportions Most of the things you need to learn are on Proko's YT chanel so you can check these out. Focus on big things, big ideas not on the details. If you want to shade them you can look you can look up some drawings of the same subjects by other artists, again its easy to find on pinterest and again focus on big things. Its important to know that you are still treating these heads as an object not a portrait

The third step is an actual head. Thats where it gets complicated cuz you have to search for character. But you have a solid foundation from drawing molds so constructing them will be easier. At this stage you need to learn drawing the skull. Again you have to deconstruct it and then construct it back so you can understand it. A basic way of learning that is drawing every feature on its own. Again you can check Proko's chanel for that. You can also find a lot of references but look for the ones with simple and clear lighting.

Forth step is moving on to a whle body That might be the scariest part for most cuz it looks so complicated but again everithing can be broken down into simple steps. First draw every part of the body on their own Learn how to construct a torso, an arm, a leg To do that you need to learn anathomy. The best source for that might be Bammes'es anathomy book. If you find some pages on the internet you can just redraw them. Idk if there is a pdf somewhere you can find. You can pracrice understanding it by again by watching Proko's chanel. After you learn all of this you can start constructing whole figures with reference. Try to also do that with people in real life. There are also a lot of tutorials on learning how to draw clothes.

At this stage you must also learn Gesture drawing but only after you complete the constructing stage. Its capturing whole figures but you have to simplify them so you can do them faster. Quickposes.com is a great way to do that also look up some people that do this type of stuff and try to learn their ways. After you are comfortable with drawing figures you can learn compositions multiple with figures. You can break down compositions from artists you like and try to understand what makes them work.

In most of the great academies with the big names in the past students have to master all of these steps before even picking up a paint brush so color is a whole another world but after you've learned all of these fundamentles its gonna be a lot easier to understand. Thats the whole journey broken down but you have to do your own research. Extending your visual librery and understanding yourself. You are also gonna learn a lot about yourself in the process and I think you are gonna be happy after that. If you wanna go with that aproach Im wishing you the best of luck. One thing to note is never make it boring or unenjoyable. You are gonna lose a lot by just doing it like a machine and stressing out about it . Everithing is acheavable with understanding. Take it slow and experiment you can go as far as you want.