There are species of aphids that live on the roots of plants, and there are some 'regular' aphids that might overwinter down there. Not unusual, but not observed very often.

my indoor potted ginger has root aphids. What do you suggest to control/eliminate them?

I just drenched the soil with aneem+soap solution but I'm not sure if neem is effective as a soil drench?

If the plants show signs of distress then control measures may be required but Root Aphids are difficult to control, although there is some evidence that Neem Oil products may work.

Here is a link that might be useful: Root Aphids