>>7784413 (OP)
If you haven't yet, start by learning general proportions of the body and basic shapes. Then draw a bunch for fun, even if you have no idea wtf is happening in some parts of the body and it comes out bad.
Then study simplified anatomy. Only learn the parts that stick out most and give things volume. For the arm for instance, you can group the deltoid into a single blob, then it's just bicepts, triceps, brachioradialis, flexors, extensors. No need to learn every single fucking little bit if it doesn't add visible mass. Make sure to do some creative drawing in between body parts to apply what you learn. Even learning anatomy surface level is enough to level up your figures.
Then later, if you feel autistic enough and want to draw hyper realistic muscular models, that's when you go learn the rest in detail.
Either way, it's a lot of putting up with it and grinding through. Learn how a body part works, do some trace/copies from various angles from both muscle reference material as well as normal people, do it until it clicks then move on to the next part. Some you'll get in a couple days, others will take weeks, it's just normal, so don't beat yourself or burn out.