Are these Porurin comics collaborations?

Porurin posts several series on his Pixiv that he credits "原作" by himself and "作画" by other artists.

pool #14075 credits the "作画" to Pumo (Kapuchiya), who tweeted about working on the series.

pool #14070 began with everything done by Porurin, but later Eiri (Eirri) started doing the "作画". They also tweeted about working on the series.

pool #14076 credits the "作画" to Yawarabi Juubee, who tweets every time a chapter is released: https://twitter.com/search?f=tweets&vertical=default&q=わがまま天使とおひとよし悪魔%20from%3Atokyonuenue&src=typd

What exactly do "原作" and "作画" signify? Are these posts collaborations, in the Danbooru definition?

Isn't 原作 the writer and 作画 the artist? Kinda like the whole Yuuji Kouji thing in this pool.

Given that this is an image site and people are more likely to be interested in other works by the same artist (as opposed to the writer), the writer often goes uncredited (if there is one).

Edit: And to answer your question, no. The expectation is that the 原作 person doesn't draw a single thing in the image.