I've bought only a few treasury bills on the secondary market using IBKR. I click to their bonds area, enter "US-T" for U.S. Treasuries, which shows a small list of short term treasuries. After I select one and how many bills I want to buy ($1,000 notional value each), I enter a limit order on a market for treasury bills. Change order if that doesn't sell, until the order fills. And then they are listed as part of my portfolio.
Short term treasuries mature by being paid their full value. So if you buy one for $990 and 3 months later it matures for $1,000 then you made $10 profit, or about 1%. Buying on the secondary market like this also incurs a commission that is $1 per bond, but might be lower in larger orders.