How is the debt to my brother paid off, what is the method for calculating it, and does my brother have a right to partnership in the trucks I bought from the earnings of the first truck, which his money contributed to purchasing?

You are oppressing your brother by procrastinating in paying back his right, and the procrastination of the rich is an injustice and forbidden by Islamic law. Your brother is only entitled to the amount he lent you, and he is not considered a partner in what you bought. You may give him something out of kindness and to console him.