The dominant partner is a fungus. Fungi are incapable of making their own food. They usually provide for themselves as parasites or decomposers. Some enterprising fungi exploit both at once. Cyanobacteria can take nitrogen gas from the air and turn it into biologically usable compounds, so lichens with cyanobacteria can make major contributions to soil fertility.
Reproduction can be tricky for a compound organism.
Most lichens are very brittle when they're dry, and some depend on just plain breakage to produce fragments that, like soredia and isidia, can be blown around by wind, washed along by water, or carried off as passengers on insects or birds. Other lichen fungi make spores. To form a new lichen, these fungal spores need to capture new photosynthetic partners after they germinate. Some apparently steal them from other lichens. Others have the good luck to happen upon appropriate partners.