Why does the outer edge of the leaf get yellow?
It is an elephant ear plant.
Certified GKH Gardening Expert
This appears to be Phyllosticta leaf spot. It is quite common. Treatment will be a fungicide. There are several that you can choose from. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
This article will help you to diagnose common issues with elephant ear plants: https://www.gardeningknowhow.com/ornamental/bulbs/elephant-ear/elephant-ear-plant-diseases.htm
This article will help you to grow them: https://www.gardeningknowhow.com/ornamental/bulbs/elephant-ear/growing-elephant-ear-plants.htm