The hard 'g' sound is the voiced velar plosive, produced at the back of the tongue and directed through the mouth.
The softer 'ng' is the velar nasal, produced at the back of the tongue and directed through the nose. It is commonly found in English words such as sing, wing, ring.
Some northern dialects append a hard 'g' to these words, e.g. sing-g, wing-g ,ring-g.
Up North singer (sing-ger) etc. sounds like finger (fing-ger) but down South singer is just singer (sing-er).