How did Sherlock know it was a Boomerang?

Also it has to be a returning boomerang to come back to the thrower as I studied over internet.

While you are correct that not all boomerangs are returning boomerangs, a boomerang is commonly expected to be a returning boomerang.

Note also that the boomerang that Sherlock and Irene imagine is specifically a returning boomerang.

Is boomerang the only device which has aerodynamic properties which cause it to go far distances and hence was very obvious?

No, but it is generally the only known thrown object that returns to the thrower in flight, which is why it couldn't just have been any thrown object.

So then how did his genius brain come up with it?

What made this case so complex to solve is that no one else was around (other than the witness who is presumed innocent and didn't witness the actual event either).

The victim clearly died from blunt force trauma, but who would've thrown something at him if no one else was around?

Unless... the victim threw the projectile himself. Since he's the only one around (again, discounting the witness), that can work.

But what kind of projectile would one throw that would be able to hit the thrower themselves, i.e. by returning back to them? A boomerang. More pedantically, a returning boomerang, but that is generally considered to be synonymous (or the default assumption).