Multiple transits through China

Aarav, your itinerary is definitely eligible for 24 hour visa free transit and you don't need any kind of visa.

You will print out your flight tickets and have those printouts with you. Also, you will declare in Vancouver that you will travel to Kolkata via China using 24 hour visa free transit. You will present them your flight tickets and after that you will be allowed to board the plane. Here, one very important detail. You will ask at check-in whether your luggage will be tagged all the way to Kolkata or you will have to collect it at some of the airports you have en route. Please, don't forget to inform yourself about this detail.

Upon your landing in Shanghai, you will approach 24 hour visa free transit counter where you will apply for a temporary entry permit to be allowed to continue your travel because the next segments will be on purely domestic flights. This permit will be granted to you definitely and you will proceed to the boarding gate for your next flight.

Basically, this is it.