How do you buy minutes for another number?

Excuse me, could you please elaborate on the process of purchasing minutes for another phone number? I'm a bit confused about the steps involved. Do I need to have access to the other person's account or can I do it through my own? Are there any specific requirements or restrictions I should be aware of? Additionally, are there any third-party services or apps that facilitate this process? I'd appreciate any clarification you can provide.