China has cut its holdings of US debt to the lowest level in two years amid trade tensions

Why does US have to borrow USD1,300,000,000,000 from China?

The question is why did China buy?

Because, you need to look into the dark spaces where governments don;t want you to look, there you will find what the media and government ministers don't want you to know. Go back to Accounting 101 in the old Pacioli Merchant of Venice days. if a merchant wanted to ensure ( insure ) exported goods would get to the destination, a promissory note ( paper surety ) was taken out by the purchaser to assure the vendor merchant would get payment for the imports on safe arrival, take that forward five hundred or so years and the old promissory note turned ( conveniently) into a debt instrument that an importing country that could not pay its debts said to its Sino mortgagees yes we will pay you some time in the future because we have such a good sovereign credit rating backed by the best reserve currency in the world :-)
And that is the current situation.
all China is doing, quite rightly, is to call in it's security advances (promissory notes aka US govt treasury bonds) made on unpaid exports received by the US.
Very simple when you understand what is really going on.
Only an old dog (ex wolf) in the game can tell you that kind of truth :-)

China bought it in open market. They could have bought EUR or JPY bonds. Why did not they ? They bought US treasury bonds instead because they pay higher in interest, more stable and safe.