Which apps are used for scamming?

Can you elaborate on the types of apps that are commonly used for scamming activities? Are there specific categories or functions that these apps tend to have? Are they designed specifically for fraudulent purposes, or are they legitimate apps that are misused by scammers? What measures can users take to protect themselves from falling victim to scams through mobile apps? Additionally, are there any red flags or warning signs that users should be aware of when using apps to prevent scams?