How does heat transfer takes place between any two systems?

For transfer of heat between any two substances or matter can be occurred by conduction or convection process.

Conduction is a process in which two objects of different temperature are placed in contact with each other. Therefore, heat transfers from hotter object to colder object until the temperature of both objects becomes equal.

For example, a metal spoon placed in a hot cup of tea.

Convection is a process in which a fluid generally liquid or gas is heated and denser material (colder) sinks at the bottom whereas less denser material (hotter) rises at the top. This causes conventional currents in the fluid.

For example, heating rice in a pot full of water.