You can find the EZ9 diagram in the PDF manual, available from both Zastava and Century.
Yes, it isn't quite a Sig clone, I've read that it's more of a blend between the Walther P88 and the Sig.
The Arex Rex Zero looks like someone really prettied up the CZ99, with better finish (not hard to do!), put a rail on it (think the EZ9, or maybe the CZ999 have that), slapped a safety on the frame, removed the right side decocker/slide release lever, and doubled the price.
The magazine has me intrigued; both the Arex and the Zastava use a Beretta/Sig dimension magazine, with a D cut on the front of the magazine. I have not cut my Beretta mags yet, but cycling by hand while holding the magazine in from underneath, it will cycle rounds and hold the slide back when empty. The follower on the CZ appears identical to the Beretta. The magazine fits precisely. If you pressed me to make a guess, I would say they are the same body, spring and follower, with different cuts per their request.
I can't tell if the D cut on the Zastava is exactly where it is on the Arex, but just eyeballing it, it has me extremely curious. It sure does look to be the case.
I wish I could find some of those super-cheap Beretta mags we used to see, I would buy a couple and do some dremel work. Anyone know of the $10 and under Beretta stash?
Added: there is a nice review of the Arex here
https://www.youtube.com/watch?v=4i9tw_Uo9L0
Decock/slide release is identical to the CZ99, but not ambidextrous. Push to release the slide, but the hammer is still back. Push again to decock. I assume (but didn't see) the safety can go cocked and locked.
The guy continues to compare it to the Sig, and that is natural; but it's a lot closer to the CZ99. I know "clone" isn't technically the right term, but it's like a Star Super compared to a 1911... you look at the gun from 5 ft away, and that's what you see. Internals don't match, different designs, but the gun definitely evokes the Sig. I think it's meant to.