@User: What do you have in mind for "low level primitives of the I/O cells"? There are things in the I/O cells that belong to an otherwise unobtainable (?) library which would be, by our nomenclature, sky130_fd_esd or maybe sky130_fd_pr_esd. As far as I am aware, these cells appear only in GDS, and maybe in CDL (I have not looked closely at the CDL for the I/O pads). Mostly when dealing with the I/O cells, I just deal with them as black-box entities.