The front closed via an invisible zipper, with the top left free and a hook-and-eye closure to hide the zipper's pull tab, therefore creating an entirely concealed front closure.
Although rare, sometimes, if you know to look for it, you can see a slight "dip" on the front where the closure tension is transitioned from the zipper to the hook-and-eye.
|VOY, 1x5 "Phage"|
|VOY, 1x1 "Caretaker"|
Also rare, but not unheard of - sometimes the uniform would pull open a bit, revealing the hook-and-eye.
|VOY, 3x15 "Coda"|
The front zipper/opening was supported with a facing, which further helped to conceal the zipper and other construction elements (such as seam allowances) whenever the front opening/yoke occasionally rolled open.
Impressions of the facing were occasionally visible on the outside of the garment:
As you can see, the width of the facing varied a bit from character to character, though it consistently stopped short of the first body panel seam.
Again referencing the screen-used uniform "LtComm" was so kind to share pictures of, we can see that the zipper/front facing was secured all the way up to the yoke facing:
This helped establish the "futuristic" illusion of the garment - i.e., "How was it made?!? It looks the same on the inside as it does on the outside!"
This can be a lot to digest, I know - good for you for sticking with me!
And if you got lost a long time ago, don't blame me ...