How is the Bt cotton plant created as a GM plant? How is it protected against boll worm infestation?

Bt toxin is a toxin produced by a bacterium called Bacillus thuringiensis. This gene has been expressed in plants to provide resistance to insects without the need for insecticides. It is created as a GM plant in that way.

Some strains of Bacillus thuringiensis produce proteins that kill insects like lepidopterans, dipterans, etc. B. thuringiensis forms crystals containing toxic insecticidal protein. The toxin exists as inactive protoxins but when ingested by an insect, the inactive toxin gets converted into an active form of toxin due to the alkaline pH of the gut. This then binds to the surface of the midgut epithelial cells and creates pores causing cell swelling and eventually lysis (death).