Why the apex is the whole thing
A nail bends at one place: roughly where the nail bed ends and the free edge begins. Everything that breaks, lifts or cracks does so because of flexion at that point.
An apex is a deliberate thickening of the overlay about a third of the way up from the cuticle, tapering both ways. It puts material exactly where the bending happens, which stops the nail flexing there, which is why a properly built set lasts three weeks and a flat coat of the identical product lasts nine days.
Look at the side profile of a set under a light. If it is a flat film following the shape of the nail, it has no apex and it will lift regardless of which product was in the bottle. This is the single most useful thing to know about gel and it is almost never explained.
Which one you should have
Nails that peel, split or bend: soft gel. It is the most protective of the three and the one that lets a damaged nail grow out under cover instead of catching on everything.
Nails that are already strong and you want shape: structured gel. Cheaper, quicker, thinner, and enough.
You want length: Gel-X. It is the only one that adds any, and it is honestly better at it than sculpting because the tip is a consistent shape rather than something built freehand in ten minutes.
What we do not use
No acrylic and no MMA of any kind. No dip powder, because the shared-jar version of it is a hygiene problem nobody has solved convincingly and the poured version is acrylic by another route. And no e-file on the natural nail, ever, which is on the sanitation page with the reasoning.