Ask HN: How do you manage users on your API business/product?

I am building on an API product (imagine something like Twilio or Mailgun) and I am thinking about how to manage users in the following areas:

- authentication and authorization

- rate limiting

- accepting payments and billing

- other factors that I haven't thought of yet...?

I am wondering if there are existing tools or SaaS for these problems, or if this is something that I should just implement myself.

Thanks in advance for your help and suggestions!