How can Amazon, Google, and Apple become the first 'trillion' dollar company when there are so many trillion dollars companies out there like SoftBank, Reliance, Tata Group, and Tencent?
When people talk about a ‘trillion dollar company' they're usually referring to the market capitalisation (basically value of all the outstanding shares) of the company.
Let's have a look at the market caps of the first three companies you've mentioned (as on 6th April 2018). All numbers in USD
Amazon 693.58 Billion
Apple: 860.34 Billion
Google (Alphabet): 708.08 Billion
As you can see, these three companies each have a market share significantly higher than half a trillion, with Apple just 140 billion short. With increasing demands for the products and services of these companies, combined with their strong growth numbers, it's easy to see why people expect one of these companies to be the first trillion dollar company in the world.
Now let's look at the market caps of the other companies you've mentioned
Softbank: 78 Bilion
Reliance: 89.5 Billion
Tata Group (including all group companies): 151 Billion
Tencent: 490.5 Billion
What's important to understand is that when you say a company is worth a trillion dollars, it's the value of the company from the perspective of the market. There are huge banks, investment and asset management companies like Bank of America, Chase, AIG, ICBC, China construction bank etc, whose assets under management runs into trillions of dollars. However, these are not the assets of the company, but of their customers and do not count towards the balance sheet of the companies.
Softbank, Tencent, Reliance are not trillion dollar companies! Apple is biggest firm by market cap which is around 800Bn usd so it is nearest to becoming a trillion dollar firm! By the way Saudi Aramco after its IPO may become the first trillion dollar firm!