How do professionals design levels in Unity? Tell me about basics plsease.

I mean how do they put rocks trees etc. When fill the ground with grass trees etc performance is really bad is there a better way to do it?

 

I like Gaia, but there are several other tools out there. You should look into Veg. Studio for good performant foliage, though unity lacks behind UE4 when it comes to foliage, maybe HD pipeline will fix this because Book of the dead is sexy as F***

 

and I one more thing I want people far away to hear the gun shot diffrently can I achive that by puting 2 spherecasts on top of each other? first one will be smaller etc player will hear 2nd sound if he is in the bigger sphere

 

I believe you would just play the sound at the source of it (the gun) and use rolloff. That should give you the most realistic effect, though of course real sound is effected by the environment and how you hear it is not solely dependent on your distance from the source of the sound.