If adding buttermilk in milk turns it into curd after sometime, then how is it possible that first set of curd is made without any buttermilk?

You can also make curds without using any available curds or skimmed milk by relying on natural bacteria present in the medium. The first batch of curd is made without buttermilk, you can follow these steps:

• Take warm milk in a bowl and squeeze the juice from half a lemon, add it to the milk.
• After giving it a gentle stir, and then proceed to cover it with a lid.
• Wrap the bowl with a cotton cloth and leave it undisturbed for a duration of 12 hours.
• Covering the bowl with a cloth will keep it warm longer. After the milk has curdled, the curd is ready to use.