What are these? Screw has two positions: fixed and loose. I can only switch between them but I cannot unscrew.
It's not supposed to unscrew.
Loose means you can push the whole plastic plug into the hole.
Fixed makes the plastic plug expand to grip. This functionality depends on having exactly the right size hole.
This type of hinge has incorporated several adjustments screws. These screws are for adjustment not for support. Many You-Tube videos will show how to install. It is known among other things as a European style cabinet door hinge.