After her second child was born, Chelsea Becker took an unpaid, yearlong leave from her full-time job as a flight attendant. After watching a video on TikTok, she found a side hustle: training artificial intelligence models for a website called Data Annotation Tech.

For a few hours every day, Ms. Becker, 33, who lives in Schwenksville, Pa., would sit at her laptop and interact with an A.I.-powered chatbot. For every hour of work, she was paid $20 to $40. From December to March, she made over $10,000.