Can a magnifying glass , if bigger enough, focus solar light to melt aluminum or magnesium ?

Answer 1

Yes, it can; and so can a curved mirror. Sunlight reaches the surface of the Earth at an intensity of about 40 watts per square metre; if you concentrate enough of that energy in a small enough area you can generate in principle as much energy as you need for pretty much anything.

Answer 2

In theory, sure. In practicality, not so much. Aluminum and mg are fairly relfective, and so the absorbance by the metal will be slight. The same type of consideration applies when laser cutting metals as well.

Answer 3

possible, but then that would defeat the purpose of a lighter.