So it is told to me that the 556 in (of course) 5.56 will hold the bolt open after the last round, correct?
Now assuming I was using the yugo mags in a 556r that have the bolt hold open
Follower what would I need to do to the 556r to have the bolt stay back and locked after I removed the mag?
In other words what part or ? Is different in the 556 that keeps the bolt back.

The follower on a 5.56 556 lifts a tab sticking out the front of the bolt catch, which locks the bolt open.  Since it's the bolt catch that holds the bolt, you can remove the magazine.  The 556r bolt catch can be manually lifted in place from the outside, but lacks the tab on the bolt catch that is pushed up with the follower.

Could the 556r use the bolt catch from the 5.56 556?

You could install it for sure.

Now if it would work with standard or Yugo BHO mags is another question. One that I can't answer as my 556R had to go back to Sig. And because I don't have any BHO mags.

AJ

I'm sure it will go in, but won't work for two reasons.  First, because the AK mag has the mag catch sticking out the back, there is a gap between the rear of the mag and the rear of the magwell when seated, and I don't think the finger on the 5.56 BHO is long enough.  Even if it was long enough, the top rear of the AK mag is too high and lacks the rear cutout in the mag body for the follower to raise the BHO.  The rear of the AK mag will always push up the BHO even if rounds are in the mag.

If you're dead set on having to have a true BHO, you'll probably have to go with the AR style 7.62x39 mags and AR lower.