Boston University alumnus Alexandra Shadrow launched Relovv in August 2018, an app that she said she hoped would make the fashion industry — the second-most wasteful industry in the world after oil — more sustainable.

Alexandra Shadrow, who graduated from Boston University’s College of Communication in 2015, has created a business at age 23, and it is growing. Shadrow founded UNItiques out of her BU dorm during...