Bats are one of the three vertebrate flyer groups. They are the only mammals among the three groups.
https://www.researchgate.net/publication/346778092_On_a_wing_and_a_prayer_limitations_and_gaps_in_global_bat_wing_morphology_trait_data
A bat has five fingers, and four fingers elongate to form the wing structure, leaving the first finger as the only one independent of the lifting surface.
The lifting surface is a membrane.
A bat can adjust the four fingers independently, leading to a great controllability of the wing shape. [1]