This is my very first autoflower grow so i'm ...

They do everything a photoperiod does, but faster. How? Think of it like this: a photo strain that does 4 week of veg (18 hours light schedule) and 8 week of flower (12 hours if light schedule) will have a total of 4*7*18 + 8*7*12 = 504 + 672 = 1.176 hours of light to power it's whole growth and bloom. Now an auto strain doing 20/4 light schedule will get, during a 8 week grow cycle: 8*7*20 = 1.120 hours of light. Roughly the same amount of light to power its growth, but 4 weeks faster. Usually the seedling / veg stage of autos lasts 3 weeks, then you get 2 weeks of stretch and 3 to 4 weeks of bloom stage. Depending on how much light the are getting they can take a few more weeks.