There's a lot of questions here, I'll try to break them up.
How is this possible? Will the yeast be able to operate in so low temperatures?
Secondary fermentation is a bit of a misnomer- typically by the time you "secondary" your beer- the beer is usually fully fermented. Ale yeast certainly will be slow moving at 8C but will remain alive and somewhat active.
How can I find the suitable secondary fermentation temperature for a given Yeast/Wort?
You can ask ten homebrewers and get ten answers. I usually go around 14C (60F) for my conditioning for Ales. I find that keeps the yeast active while helping with flocculation a bit.
What is the purpose of secondary fermentation in Belgian type Beer? Some sources highlight clarity while others mention the flavor maturity. Is this process needed?
This is a brewing argument that never ends. Some people never secondary, some do. My guess is the practice is adopted from breweries where they dump their yeast slurry after primary, and homebrewers try to mimic this. Secondary fermentation this really just a conditioning/aging step where you want your beer to continue to condition but you want to avoid yeast autolysis. I'm not convinced it helps with clarity but it can somewhat reduce trub in your bottles.
Is this process needed?
If you're going for a straightforward Belgian beer, I think that you could easily go from primary fermentation to the bottle without much of an issue. If you plan on extended aging, or adding adjunct spices, secondary might make sense.
The only similar step I have followed is to add sugar while bottling and let the beer be carbonated naturally. Although the yeast does operate during this stage, this is not secondary fermentation. Correct?
Correct, this is a fermentation, but not what we colloquially refer to as "secondary fermentation".
FWIW I don't think the brewing process for "belgian" style beers and any other typical ale is especially different. Some folks like to crank up the primary fermentation temperatures a bit, though my personal taste is not to do so.