Is a person obligated to finish the food left over by others in their plates, and is he sinful for discarding food remnants that do not belong to him and he does not wish to eat?

It is not obligatory to eat what remains on another person's plate. If the amount is small, it is recommended to finish it. If it is large, it should be given to someone who can benefit from it, whether human or animal. Discarding small amounts of leftover food is not forbidden. However, large amounts should either be eaten, given to someone who will eat them, or thrown to animals for their benefit.