In short, the government doesn't just go to the next local branch of a bank and asks for a loan. Instead, it asks "the public" for a loan, resp. for many "mini-loans" which can be bought by individual investors. I don't know their exact minimum in India, but in Europe, they can be as low as 1000 €. These loans (also called "government bonds") can be due in a short time (as noted above), others can be due after many tens of years. But that's no problem - they can be bought and sold at the secondary market (at exchanges).