John Gill Commentary Exodus 28:25

"And the [other] two ends of the two wreathen chains thou shalt put on the two settings, and put them on the shoulder-pieces of the ephod in the forepart thereof." — Exodus 28:25 (ASV)

And the other two ends of the two wreathen chains thou shalt
fasten in the two ouches
In which the two onyx stones were set on the shoulder pieces of the ephod, and were as buttons to them; probably there were rings to those ouches, into which these ends of the wreathen chains of gold, reaching from the breastplate, were put; or however, by some means or other they were fastened to these ouches or sockets;

and put them in the shoulder pieces of the ephod before it ;
that is, on the ouches upon them, as before observed: into that part or side of the ouches which was to be the fore part of the ephod; so that the breastplate hung by these chains from the shoulder pieces of the ephod, on the fore part of it, upon the breast of the high priest.