Foreign Investment

I read in an article:

"When asian banks buy government debt in the form of T-Bills, it helps keep interest rates at an all time low even as the united states has massive deficits."

How? Why?  Is it because the foreign investors have to buy american dollars in order to invest in american t-bills, which increases the exchange rate.. but how does this affect the interest rates? Does it just mean that they have no incentive to raise them since they do not need to attract extra investment?

But then I'm also confused because if the government sells securities in the form of t-bills.. doesn't it mean that the money supply goes down since the government is collecting this money, therefore causing the interest rate to go up? (In the money supply/money demand model)

Thanks