Is the plot of land that a father gifted to his three children, upon which they built with their own money, considered part of the inheritance after his death, or is it not included within it?

If the father's offer of a plot of land to his children was an immediate gift and was acquired during his lifetime, then the children to whom it was gifted own it. However, if the offer was a loan (عاريّة) of the plot of land, then it is considered a loan from the father to his children, and no heir has exclusive rights to it over another. And the father's permission for his children to benefit from his property is not to be interpreted as a gift unless explicitly stated. Therefore, the father's statement to his son, "Build a house on this plot," is not considered an act of transfer of ownership. Since this matter refers back to custom (عرف), the reference for it is the Sharia courts.