you have to mix in iron powder and other metals, graphite into the paint, thats metallic paint, the more metallic filler, the better. it is best to combine the paint with actual metal sheets, laminated transformer cores are the right material for EM shielding, but buying the alloy in the form of sheets is not affordable, you have to use cheaper metal foil/sheets, but use more layers and sheets of higher thickness, a single sheet of annealed transformer core will shield more EM than 2 inches of standard steel, i belive it is best to combine different ferrous and non ferrous alloys, like transformer cores.

Yes. Drapes and fabrics for this sort of thing does exist. They are fabrics that have metal threads woven into them for the shielding effect. You can search for just "EMF sheilding fabrics", and you should find them. Paints also exist. YShield and GeoVital paints are two examples. It's quite expensive stuff though.

When 5G eventually gets to the higher frequencies, then even aluminium foil will completely block it. At 2-5GHz aluminium foil only attenuates. You can add layers to achieve better blocking. At 20+GHz I doubt it gets through walls at all. Windows probably will still need to be shielded.