APOPTOSIS

female hair is a lil bit tricky cuz they have more creativity with their hair yeah? so divide it into regions: bangs, middle (which can be further separated to two more parts), sides (easiest to deal with) and bottom (this region decides the flow of the character's hair, like straight, wavy or curvy hair)

I feel female hair usually have more volume than male, particularly because of the bottom part, so keep that in mind!

Volume

I struggle with this myself so err take this with a grain of salt. firstly, to make a face look less flat, you need to learn about face plane. it's basically a 3D head divided into section to show the indents and raised of a face. i obviously doesn't follow every line because the human face has more meat, so the lines i choose usually involves the area around the eye, nose and neck. you can define it with a few simple line or shade your character like so:

(this is more applicable to non-chibi) the angle in which you draw the character in also play a massive role to show how much volume a character have. if you take a look below, you can tell the middle face (front facing) looks "flatter" than the one on the left and right. (idk what's the right term but) "Angular Lines", represented by the number, is something that helps define shapes to me. i feel like if there's lesser, then the character will appear flatter. so that's why you'll notice why i usually draw in 3/4 angle (the face on the left).

when it comes to chibi, since the faces are usually drawn with curves and roundness, the angular lines doesn't really work. the grooves line kinda works too but with chibi, the "squish factor" is what determines the volume of a character. Looking at row three, if there are curvature on a face, it looks squishier right? try to imagine chibi heads as squishy dolls, exaggerate their face proportion by pushing that curve.

Proportions (chibi)

this is the proportion I use, usually the head is the biggest part of the body. the bigger the head, the smaller the body, the more gremlin energy the character exudes. For the arms and leg, I just draw it as one part instead of dividing it into two (like legs have thighs and lower part, and arms have the biceps, forearm and so)

this is entirely up to your style, but i find wider, chubbier chibi is cuter over "normal" sized chibi (or I usually call it Longer chibi)

some example for your reference on what I mean:

chubby chibi vs longer chibi

I also wanted to include these images for the body proportion, Im more on the left side with the square cuz the cod characters have those body shape (taco1704 has many great tutorials for chibi and non-chibi, you can either find it on their twt pages or pinterest)

Proportions (non-chibi)

another weakness of mine, but usually a full body proportion is about 8 heads. i'd advice focusing on each part rather than practicing a whole body at one go, meaning try drawing from the head to the chest first, familiarize the distance of the chin to neck for example

for the face I usually divide it like so. the distance/gap between two eyes is one eye. but keep in mind with different face shape, the numbers would change, such as the forehead, eye, nose size, length of jaw and roundness of chin.

here's an example of changing certain part of the face to have more character variety

back on the dimension thing, to make a pose look less flat you need to consider line of action, it's basically the overall shape and line of a character. look it up on pinterest and you'll get what I mean. Curvier lines with more angles looks more alive. For chibi, think of the squish factor again!

some creators i recommend (I learned the concept and then apply it to non-chibi):

Bluebiscuits

sinix (very good foundational videos, it's ok if you don't get it though because it can be a bit advanced if you just started drawing)

emiliodekureart (their videos are literally the foundation on my anatomy, it's simplified and straightforward!)