I really do not understand how bodies work in this…(manga spoilers)

She probably found a suitable body in between things? There were sixteen years between this and the blue night. The chances of finding one isn't that out there, or she probably has had this one for a while similar to Mephisto. Lucifer burns through bodies because of how much power he has. Iblis is a demon king but she's also lower in power compared to him so her bodies aren't going to be destroyed as fast as his would.

The status of the Demon Kings is as follows.

Lucifer - Repeatedly cloned bodies while transplanting his own head.
Mephisto - Uses time magic to halt the decay of his own body.
Azazel - Allowed his body to crystalize and return to nature.
Iblis - Has seemingly not changed bodies, though there are some slight physical differences so possibly has.
Egyn - Has likely changed bodies, his current host may well be a clone from the same group as Iblis.
Astaroth - Previous body has seemingly failed, currently possessing Reiji Shiratori, a non-clone.
Amaimon - Possessed a body from the Samael group in Asylum.
Beelzebub - Unclear, but has apparently possessed a new body in the last 100 years.

Now to be clear, it's not that Amaimon's was the only success to come out of Asylum, but successes were rare due to how the cloning system worked and Amaimon was seemingly the last one to occur there before it swapped over to full scale Elixir research. Since then, we know that the Illuminati took over whatever research was being done at Asylum, with Drac Dragulescu creating a secret area in the Romania branch and producing the Lucifer clone bodies and the surgical system Lucifer uses currently. Along with the remaining clones from the Lucifer and Azazel groups that were secreted there. We also know that, while not Demon Kings, Amon and Asmodeus also claim to have new host bodies already lined up for them when they fought the True Cross Order, so its entirely possible that the Illuminati have taken over the practice and have provided bodies for Iblis and Egyn since Asylum shut down.

With that being said, its noted that the weaker Demon Kings don't burn through bodies as quickly, with Lucifer's lasting less than a decade, but Amaimon's currently lasting more than 16 years without degradation. So it could simply be that Iblis as the middle of the Demon King pack has yet to burn out the body she had pre-Asylum's closure.

As we see with Astaroth as well, it's not impossible for a Demon King to find a human host who isn't a clone, so it could also be that Iblis and Egyn's current host bodies aren't clones and are simply human vessels that were compatible enough.

I just want to point out that Egyn has a body inside of the shark, the question is if it's a shark onesie or a cadaver?

the bigger question is was that body banging BEFORE iblis possessed it or does she turn every body she posses into a babe