> We are not comparing data usage, we are comparing the value of two different currencies. The value of 100 US$ in India is different from that in the US or the EU. In order to normalise the value between two currencies, PPP is used. It doesn't matter what the 100US$ is spent on, data or food.
But you are comparing value of currencies by comparing two equal goods. That would mean also comparing acquisition of raw materials, labour costs, import/export of goods, availability of resources etc which is never going to be the same. PPP itself has shortcomings (you can read them here: https://www.yourarticlelibrary.com/international-trade/15-cr...).
For example, India might have abundance of rice while USA might have shortage of rice. But rice is used for making noodles in both countries and demand for rice is equivalent in both countries. Would you price it according to PPP index? Nope. You take into account any tariffs imposed by host country. What about transport? What about labour costs? There are so many variables that can influence the pricing of a product. PPP is a bad way to judge cost of living and cost of goods/services.
If PPP is 21.3 (2019 data) between USA and India, a Harley Davidson in India should be 21.3 times cheaper than a Harley Davidson in USA. It isn't. The cost is pretty much the same. So is the case with iPhones. Even if iPhones are manufactured in India it would still cost slightly less than in USA. It will never be 21.3 times cheaper.
Also, let us take Big Mac Index into account. It was a humorous take on PPP devised by economists to compare the costs of a Big Mac between countries (as McDonalds maintained standards between countries). Now the cost of a Big Mac in India and a Big Mac in US might not reflect the true PPP of the country because it all depends on the appetite of Indians for a Big Mac. We are avid consumers of street food. Street food for us is what is McDonalds for Americans. Now how can you make McDonalds as a standard for the World when in my part of the World a tiny percentage of the population visits McDonalds to consume their delicious burgers while a large percentage eats street food? Wouldn't that obviously influence the price of a McDonalds burger in India as the demand for it is next to non-existent? Now let me reverse the case and say that India tomorrow undergoes a massive cultural change and everyone shifts from eating street food to eating at a McDonalds burger joint. What would the price of a Big Mac in India be then? Obviously with high demand the cost falls rapidly. PPP should blow up right? But does that truly reflect the standard of living between two countries? That is a food for thought for you.