Oooh, yay! I love it when people are interested in learning to draw --- particularly when it's for the purpose of making more Omo! XD
I don't know if I'm the best representative when it comes to manga, or art in general, though I do try to mimic its various styles in my own stuff... That said, I like to ramble about art, and I've got a little formal training in the form of college-level drawing courses!
For drawing characters, there are a lot of different approaches, and many of them might not work for you; I've seen tons of different art videos where the teaching is solid, but it just didn't really work for me. So you might need to "shop around" a bit to find someone whose teaching style ends up working for you! You can usually tell if you're happier with the result of following their instruction, and if you are able to apply it more broadly --- like, you can use it to draw a different character in a different pose, but the tips still help you produce a result you like.
There are a few things that tend to carry through all of them, though! Humans are really complicated and dynamic shapes, and our brains are really adept at finding all the little things that can be "off" about them --- hence why they are so hard to draw. One of the most solid foundations for drawing them in any style is to learn proportions and anatomy; as the old adage goes, you need to know the rules before you can break them! And you most certainly can break them --- but there's definitely a need for some foundational knowledge, or everyone would be a master right off the bat! Also, reference is always good --- I think our minds tend to delude us into thinking we know how something looks when really our minds are more like a book of "symbols" that we've learned to recognize of our lifetime. When it comes to characters, and in-particular manga, it can be really helpful to see how previous artists have "solved" different problems, such as how they define musculature or different joints via outlining or shading.
Because Humans are incredibly complicated, it also really helps to break them down into extremely basic shapes before trying to pile on detail. I know a lot of people have probably told you to "learn anatomy and proportions" (as I have) before, because it's definitely true that you'll need some understanding of those to make you characters look "right". But that doesn't mean you need to know every last muscle and ligament in the human body to get something you're happy with! There's no point in overwhelming yourself --- particularly if you aren't already drawing characters you're at least somewhat happy with. In fact, a lot of my own drawings start with stick figures or scribbled "gesture" drawings! It's easier to "correct" a stick figure that exists than it is to rip a perfect human shape from the void, lol!
Anyway, if you're interested in trying out a few YouTube channels to see if any of them help, I've got a few recommendations!
https://www.youtube.com/@mikeymegamega --- Mostly specializes in sexy manga ladies, but I think he's done other stuff in addition. More importantly, though, he tries to explain how the various shapes you're drawing are supposed to be 3D. This is essential if you're trying to draw characters in different poses and at different angles, but I'm not sure if his teaching always conveys this in the most effective way (at least for me).
https://www.youtube.com/@WhytMangaOdunze --- Excellent capture of the manga style, though his videos range outside of instructional stuff a lot of the time. He also provides a lot of good tips regarding laying out panels for pages, and so on. He also has a good number of videos where he's just drawing, and it might be nice to just watch and see if you can figure out how he was able to "get" to his results.
https://www.youtube.com/@markcrilley/ --- And old member of the Youtube Manga Tutorial community, and a pretty good one at that! He also has a lot of interesting "meta" content about artwork, like audience-Q&As and so on. He's a big advocate of mixed physical media, so if that is your thing, I think you'd enjoy his content. I also think he's still been improving steadily over his time on YouTube. That said, I think some of his tutorials tend to fall into the trap of being "one-off" instructions... It's kind-of like when you see those old tutorials explaining "How to draw C", and it's just a step-by-step from basic shapes to completion --- basically they show you how to draw that one thing in that one way, but don't really explain the underlying forms that would allow you to draw an object from any angle. He might have gotten better, though, but I haven't watched his stuff in quite a few years.
https://www.youtube.com/@RODGONTHEARTIST --- I've been watching this guy's streams a lot lately, and while his stuff doesn't always hit the mark for me, it does a lot of the time! He's more of a western cartoonist that someone who specializes in anime, but if you see his streams, I think you'll see how his advice can be applied more broadly. In particular, he stresses the need to understand 3-dimensional shapes, and he does it well. If you want a great example, give this video a watch:
(The thumbnail isn't really the greatest representation of what he can do, lol.)
The downside to his videos is that his instruction is essentially formatted as a long stream, so it tends to hop-around a bit to follow viewer comments, and it can take a lot of time to get to the good stuff.
I'd cautiously recommend https://www.youtube.com/@ProkoTV. He's got tons of great stuff on his channel, but it is definitely geared toward the fine art audience. That is, he'll really go deep into the little details about anatomy, and it might be too overwhelming to make use of if you're just starting out!
There are tons of other channels on YouTube that might help you, but these are just a few that came to mind --- again, some might work, some might not, and neither instance necessarily has anything to do with the quality of the content or your own ability. It's just a matter of finding something that gets you to a result you're happy with! I'd also say that you really don't need a drawing course, since most of that stuff is covered in the first chapters of any "how-to-draw" book --- types of shading, using shading as opposed to outlines to define form, different types of perspective, etc. 90% of those courses is spent practicing the newly addressed subject anyway. One take-away from them, though, is that sometimes you want to study a real object instead of a picture of one on your screen! Your screen is flat, and the picture has already "solved" projecting a 3D object onto a 2D surface --- this can be detrimental when it comes to understanding the 3D forms that make up a given object! I can't speak for a figure drawing course, I should add, though.
Anyway I don't know if any of that will be helpful for you, but I hope some of it proves useful! I'm pretty much always down to talk about art and how to draw, so if you (or anyone else) wants to ask me something, go right ahead! I'll do example sketches if it's necessary too!
Also, depending on how willing you are to share them, I'd love to see where you're currently at when it comes to drawing characters! Sometimes stuff you think is bad isn't, or it might be pretty close to something you'd be happier with!