What emails do I need to setup for my side project?

I'm working on a personal finance app, and I'd like to know what emails I need to create to send to my users. I think I might need a welcome email and an onboarding one and probably an upsell one.

I'm curious how you guys approach emails and any tools that you might be using to set them up. Also if you have any templates that you use, I'd appreciate it. Thanks!!