What is the most effective way to gain muscle mass? are there some exercises that actually lead to a drop/reduction of muscle mass? e.g. do aerobics?

Use it or Lose it.: The most natural way to build muscle mass is to use that particular muscle. In general, our bodies perform efficiently when they are being used. This is the same for muscles. Gradually building up workload on a particular muscle with proper hydration and nutrition is the most effective way. There are pharmacologic therapies that can also help, but are not clinically indicated for your goals.