During the initial brining I don't believe there's actual fermentation happening yet. But by doing so in brine, absorption of salt via osmosis makes the salting process even all through out the mango vs surface salting, the exterior ends up being way saltier than the interior, Unlike daikon, cabbage or cukes, unripe mango cell walls are too rigid to accept the coarse salt as it is, you'll need a "vehicle", in this case water, to introduce it to all available surface area.
During the sugar solution part is when the fermentation actually starts. As far as how active it can be, depends on your ambient temperature. My house temp is 72 year round, cooler than what most people prefer ( I have planted aquariums, and most of the plants I have tend to melt in warm environments ) & sometimes fermentation takes longer than what is normally suggested so I go 2-3 days longer. My parents keep theirs at 76-78, most things ferment "on time" at their place. You'll start seeing tiny bubbles appear within 24 hours. I suggest leaving some headspace of an inch and a half to two to prevent it from bubbling over. Within day three when you burp it, you'll definitely start to smell signs of active fermentation. At day 7, it will smell like fruit wine, but still not alcoholic enough to make one intoxicated. It's time to slow/halt the fermentation and stick it in the fridge, test one, it should be crunchy like a pickle should be, if it turned mushy either almost ripe mangoes were used, or it wasn't salted properly in the beginning(fail).