Where do aphids come from?

My Coconut Plant (Scented Pelargonium Grossularioides)
seems to be growing healthy, until few days ago ...

... when I remarked whitish-greenish bugs all over the back of the leaves.

I studied a little how to get rid of them without killing the plant and how to protect my other plants, without using harsh insecticides. I have many edible plants in my garden and I like to use those when I cook . So, I made one natural insecticide (I don’t want to expose myself and my family to chemicals).

After I browsed various websites, I made one natural insecticide using household ingredients like garlic and mosquito plant leaves which target insects like aphids, caterpillars, Colorado beetles, white-flies and soft-bodied insects.

I combined in a blender 5 leaves of scented Pelargonium Citrosa, tobacco from one cigarette and 1/2 bulb of garlic. Boil 1 liter of water, add the mixture and let it stand for 2 hours.

I sprayed the infested area (in the cool of evening)  with this magic mixture (better to do this in the early morning or cool of evening to prevent a negative reaction when excessive heat ).

In the morning, my Coconut Plant seems to be healthy again.

Still I'm wondering and few questions persist in my mind: Where did aphids come from? Will disappear after this treatment with garlic and tobacco tea or should I repeat it after a while? What can we do with them?