What are your go-to solutions for authn/authz and payment processing in side projects?

In the realm of full stack development, implementing authentication and payment systems can be daunting. Many developers face challenges like integrating social logins (OAuth), managing backend permissions (RBAC), and ensuring secure payment processing. What tools and services do you use for authentication and authorization? Are you opting for third-party solutions like Auth0 or Stripe, or are you building your own? Share your experiences and any resources that helped you along the way!