The purpose of an expansion joint on a bridge it to permit the components to expand and contract with changing temperatures without stressing the entire bridge structure. Early bridge failure would be the result of building a modern bridge without expansion joints.
Absolutely not. Property held as joint tenants passes automatically to the surviving joint tenant. The decedent's interest in the property disappears at the moment of death so there is nothing that can pass to their estate. That is the purpose of "survivorship" in a joint tenancy.
The overall purpose of a joint is to provide movement . Differing types of joints allow for differing movements; a hinge joint allows motion in one direction whereas a pivot joint allows rotating or twisting motion.
A joint account is just another bank account which can be used for any purpose. Yes, you can use it to take a personal loan or pay for one. There are no restrictions reg. joint accounts not be linked with personal loans.