What functions do fungus and algae perform in the body of a lichen?

What functions do fungus and algae perform in the body of a lichen? why is lichen called symbiosis? Why can lichens live in the most barren places?

Lichen is a symbiosis of algae and fungus. It is because of this feature that the lichen can be attributed to two kingdoms at once – mushrooms and plants.

This organism can be found on all continents. The presence of this organism everywhere, even in those places that are unsuitable for the life of other organisms, can be explained as follows: the alga is an autotrophic organism, that is, it can synthesize organic substances, and the fungus, the heterotrophic part of the lichen, uses these substances for its own existence.

The function of the fungus in this symbiosis is to absorb water with minerals, and the function of the alga is to form organic matter through photosynthesis.