What is the ruling on zakat for a plot of land purchased with the intention of investing in it, whether by selling it or building on it for rent, and what is the ruling on zakat if it is transformed into a building for a rental project?

If you did not intend for the land to be for trade when you purchased it, then there is no zakat due on its value. If you were hesitant in your intention, then zakat is not obligatory. However, if you built upon it for rental properties, there is no zakat on the buildings themselves. Zakat is only obligatory on the rent if a year has passed over it and it reaches the nisab either by itself or by combining it with other wealth.