"I'm curious to know why cycles are so short (8-10 weeks)"
Many people will suggest different ranges of cycles, but 8-10 is the norm as of now because many fear that htpa recovery will be very hard above that point.
"why steroids take 2-3 weeks to take effect"
They take affect from the second you inject, but you won't feel them till they accumilate in your system. With orals it can be as fast as the first week, with long acting injectables as long as 4 weeks.
"what exactly is the steroid doing within the first few weeks"
Building up in your system if it is long acting.
"how at the 2-3 weeks mark you get the sudden surge of strength."
Look above.
"Then what happens as you get further into your cycle and why must it end at around 8-10 weeks?"
You keep gaining as the cycle progresses, and you even gain after the 8-10week mark, depending on the ester of the steroid you took.
It doesn't have to end at the 8-10 week mark, but it is a good rule of thumb if you are just starting IMO. Some people will suggest much longer cycles and a others will suggest 2-3weekers.
-sk