[twawaaay]

Imagine you have two very long tubes joined thermally throughout their entire lengths. For the sake of simplicity, no energy is exchanged with the environment, only between the two fluids.

The cold fluid will meet (thermally, in adjacent pipe) more and more hot other fluid and finally as it reaches the end it will meet the hot fluid at exactly the temperature it comes in. If the fluids are flowing slow enough and the thermal bond is good enough, the cold fluid can get heated up as close to the hot fluid temperature as you want.

And the hot fluid will meet colder and colder other fluid until it reaches the coldest at the other side.

The efficiency of heat exchanger could be as close to 100% as you want, in ideal world.

In real world higher efficiency requires larger device and there is a point of diminishing returns. You also get other losses like mechanical losses due to need to pump fluids through pipes (with large surface area), due to need to have turbulent flow (to ensure mixing within pipes) and due to heat loss to the environment.