what is thermite reaction?why magnesium strip is used and not fire?

A Thermite reaction is basically iron oxide (rust) that reacts with aluminum to produce molten iron. The reaction is triggered by the heat of the burning magnesium band being put into the iron oxide / aluminum mixture, that's the fuse! (Any source of heat can be used to start the reaction, but it must be very hot, since magnesium burns at around 650 ° C (1200 ° F).) A sparkler might work, but I did not try that.)
Because......This reaction is highly exothermic and produces molten metal at about 3000 ° C.
Water should not be used to quench the reaction as the addition of water to hot iron will potentially produce explosive hydrogen gas.