how atmospheric inert nitrogen gas is converted into usable nitrogenous form for various life forms?

The conversion of atmospheric nitrogen to usable form is done by the process called nitrogen fixation.

Leguminous plants are those which have nodules in their roots which can absorb the atmospheric nitrogen. This process is called nitrogen fixation.

The bacteria present in the leguminous plants contain nitrogenase enzymes which helps the reaction between hydrogen and nitrogen to produce ammonia.

This is how nitrogen is trapped in the soil by which plants receive nitrogen and animals eating these plants in turn receive nitrogen.

This is how atmospheric nitrogen gets converted to usable form and help plants and animals to in take nitrogen for their living.