Starting from right, Akane seems to be the most skilled at it, being able to hold her weight steadily and striking an erotic posture simultaneously. She is using two fingers and is not ashamed of her activity at all, which fits her open nature and indicates that she might be enjoying such procedures regularly.
Aoi, on the other hand, is barely daring to use direct contact which is still far from the most important spots and has mostly relied on indirect rubbing, thus showing her probable shame of revealing, or making contact with the spot in question throughout her youth. The theory of her being more reluctant than usual due to the activity happening in a group is undermined by the fact that she has effectively achieved an output no lesser than that of other participants with minimal effort in comparison to theirs, indicating that her involvement in such process occurs less often than among her peers. She however is not shy of this, possibly because of not being aware of her surroundings anymore due to her state of mind at the moment, which also indicates her inability of controlling herself at such situations yet, possibly being completely new to the sensation and even a virgin.
Wakaba is in fact managing an even better balanced posture than Akane, possibly due to her professional skill in martial arts, yet falls behind in terms of her actual experience. As we can see, she is not only the most flustered but also using only one finger for entering the cavity, as well as showing insecurity of how far should it go. She has some experience with the process itself but it seems to be minimal.
Finally, Himawari seems to be needing a whole fist and a thumb to fill it all, giving us evidence of how much free time without social contact she has had under those warm blankets she basically lives in. Her partially childish innocence and complete immersion in one single process in her eyes also corresponds to her lack of social experience and autistic nature.