Internal wastegate is installed in the turbo set up it self and the external wastegate is a separate unit. The purpose of a wastegate is to regulate the amount of boost...
The internal wastegate is setup that on the exhaust side there is a little valve and the wastegate opens it and lets exhaust gas flow directly out to the exhaust system or it stays closed and the gas is used to charge the air and spin the turbo...Correct me if i am wrong