I don't mean to criticize, mind you, so take this as a pattern Im seeing.
You like to post a lot. That's good. But you ask vague, generic, and really open ended questions with no right answer most of the time, and for the most part could be answered by most books or even internet sites.
Again, don't mean to be unkind, just wondering why all the silly questions?
Re: fights: Well depends. Early on alt was king. The spits turned well but the 109s had the edge on horsepower at alt. 25k wasn't unheard of. Later on the bombers kept going higher, so the fighters had to follow. 30k wasn't unheard of. That's why the dora and the 109K and the ta152 were developed -- the high alt performance they brought to the field.