The way that plants acquire nitrogen is through their roots from the soil.
Plants get their nitrogen through their roots as amino acids, nitrate ions, nitrite ions, or ammonium ions.
The soil is where the plants acquire the nitrogen they need to survive and thrive and they do not get the nitrogen through the air even though the air contains nitrogen.
The nitrogen that plants get and need comes from the soil where it has already been fixed by bacteria and archaea.
When animals eat the plants then the animals get the nitrogen inside their bodies through eating those plants that have absorbed the nitrogen.
Even when humans eat plants you get the nitrogen from those plants as well.