As isthmus is a narrow strip of land that is bordered by water on each side, and that connects two larger pieces of land. how does this differ from a tombolo?

An isthmus can exist between two large landforms or between an island and a larger landform or continent. It is any narrow connecting neck of land.

A tombolo is a sandbar that connects the mainland to an island, forming a permanent or temporary isthmus, and the island can be known as a "tied island". Tombolo's form from sand or sediment pushed along a shoreline.