1099-B doesn't have cost basis and this amount was used for sell to cover my taxes on RSU.

I am trying to fill in my taxes and I imported the 1099-B so far so good. At the end with the 'smart check' feature, it says there is some information needed and it will ask me for a sold date, proceeds, and cost basis. After I enter cost basis my tax return goes down by a huge amount. Now I have few questions about this:

1. The stocks that are sold are for covering my taxes then why am I paying taxes on this amount too?

2. In my tax document there is 1099-B section and also a supplemental section. The 1099-B doesn't show cost basis value but the supplemental shows both proceeds and cost basis. Which one should I use?

3. In the 1099-B section where there is a value present for 'proceeds' the description there is 'Short-term transactions for which basis is not reported to the IRS'.

Thanks.