The ampere is a measure of flow of charge per unit time. Talking in terms of "amps every x minutes" does not make sense.
From reading online sources, understand 3600mAh is 3600mA per hour but still don't understand @0.2C.
No, it's 3600 mA for one hour. 3600 mA × 1 h = 3600 mAh.
Or 1800 mA × 2 h = 3600 mAh.
Or 7200 mA × 0.5 h = 3600 mAh1.
My question is, If I decide to use 5 * 18650 cells in series, 3600mAh @0.2C discharge for one BQ77915 device, is this 3.6A * 5 = 18A every 12 mins?
Again, "18 A every 12 mins" doesn't make sense. Instead, since all five cells are in series they will all pass the same current and have the same Ah capacity. 3600 mAh @ 0.2 C = 3600 × 0.2 = 720 mA discharge rate. At 18 A the discharge rate would be \$ \frac {18}{3.6} = 5\ \text C \$ (which would be 25 times the 0.2 C discharge rate).
The fact that there are five cells in series will raise the voltage to five times the cell voltage but does not increase the current capability.
1 I'm simplifying here. At higher discharge rates the battery becomes less efficient so effective capacity is reduced. Hence the 'C' rates.