How are concave mirrors used in telescopes?
ALL mirrors in telescopes are to collect/concentrate and focus the received light.
A combination of concave and convex mirrors and lenses is used to collect a larger area of light and focus it onto a smaller, possibly brighter image surface.
Concave mirrors in particular would be used in the collection and focus of the initial light received. They are the limiting factor in optical telescope design and construction because they are the largest optical component of the telescope.