How does it work, how do I enable it?

It's enabled from the start, it places a radar that keeps the roboport's construction area revealed under every roboport you build, as long as that roboport is powered.

It only reveals the construction area of the roboport, it does not scan (reveal distant chunks), you'll need a real radar for that.

The point is so that you can issue build orders to any area of your base that can be reached by a roboport.

In what cases should it show the construction area? Maybe when the item is in the hand?
Nothing changes for me. Maybe it's because of the Braver New World mod.

Perhaps it would help if I use the same language as on the radar page?

The radar that is added to a roboport constantly charts chunks that are within the construction area of that roboport.

It does not participate in long-range survey scanning.

The purpose is to provide coverage of the area so that you can deploy blueprints or order deconstruction of anywhere that a roboport can build.

It has no effect at all on anything that is not a placed roboport connected to the electricity grid.

Думаю, поможет, если будешь использовать русский язык — я русский. )