Is zakat obligatory for someone who borrowed money and used it to buy cars wholesale, then sold them, made a profit, bought real estate with the profit, and donated a portion of the profit? And how is zakat calculated in this case?

You are not required to pay Zakat (definition) on the real estate in which you invested your money if it is not intended for sale. However, if it is rented out, Zakat is due on its rental income once a year has passed over it. The Zakat year is calculated from the time of the contract, regardless of whether the rent was received in advance or in arrears. What you spent on building the mosque and charitable projects is a righteous deed.