From the looks of it, the mc bought special clay. Used said clay to make mouth, pussy,and boobs and some how linked them to different girls. He tested out how to swap bodies when you see the fat guy and hot girl. The last part is he probably used the clay to make his body the same as the first girl he tested out the clay on, keeped his penis and fucked her. That's what I can gather from looking at this.
Very belated FYI: The "a" you see in the title stands for "alpha" which means it was an unfinished version of the story from the author's Pixiv Fanbox page. The finished story is back on their Fanbox for a limited time as I write this comment, otherwise here is the link to the doujin version of it:
@dragonknight745 has it pretty much exactly correct: MC has special clay that can link to the target it's modeled after, which he uses to take out his frustrations on girls in his class he deems to not be nice people. In the 3rd scene he gave the black-haired girl larger boobs that were very sensitive, then he gives the brown-haired girl a dick suddenly. Curious what would happen if he swapped parts, he tests with the otaku and the girl in the 4th scene and it doesn't turn out exactly as expected: They now have each other's body from the neck down but everyone else sees them as normal. Finally, the MC has interest in how girls feel, so he changes into the first girl and masturbates. Then he goes to her house to wait for her to come home and tells her he was sprung forth from her lust and knows she masturbates everyday, etc. (I think the implication is with the clay models he can watch them like a hidden camera) She then gets addicted to "making out with herself" and begs for dick.