To solve the world's problems, just build your own world as you like

How much matter would it take?  And where would it come from?

Let's assume that you have a torus ( think do-nut)  which is 1000 meters in diameter, with a thickness of 100 meters.  Assuming an outer skin or shell of 1 cm thick, this gives a mass of 170796 tons of iron.

But you would probably not want to use iron all around.  Or iron at all.  A better choice would be stainless steel, which would not rust.  The moon has plenty of iron and silica for glass.  The new man-made world would be made primarily of these two components.

If you use fused silica glass half and half, the weight would be reduced by half, then add back 27% of the missing half. giving a result 85398+23860 equals 109258 tons.  If you wanted a thicker skin, double the thickness, and thus double the mass.

How to assemble all of this?  If you lifted 1000 tons at a time, you would need 109 launches, probably from the moon.  If you fused the two pieces together on the ground, then launched, it would still require a tremendous amount of lifting power as well as significant assembly still required in space.  You would have to fit the 109 pieces together in space.

Not to worry.  Parkins' device is claimed to be able to lift as much as 20,000 tons per launch from Earth.  Besides, the moon's gravity is only 1/6ths of the Earth's, so that's doable-- at least in theory.

With a potentially mammoth launcher, you could divide it up into just a few sections.  Each would be lifted off the lunar surface, docked with each other, and finally secured to each other.  A new world thus constructed.

Time to work to make it a livable habitat.

It would be spun at 1 RPM in order to give artificial gravity of 1 g.  You would start working on the inside so as to make it livable.  It would need shielding from cosmic radiation.  Plus a way to grow food and keep water.  And so on and so forth.  The energy source can be the sun, as it shines eternally in space.

You could build as many of these worlds as you wish.  With cheap access to space, people could start populating it.  The new worlds could be placed in Lagrange points.  There are five of these around the Earth-Moon system, plus five more around the Earth-Sun.  You could also place these around Mars and Venus.  Plenty of places available.

The sky is literally the limit.

Update:

One of the basic ores on the moon would be ilmenite  With that resource, you can make iron, oxygen, and rutile ( titanium).  Another basic ore would be silicon dioxide, or just plain old beach sand.  You can make glass out of that.