There are SaaS solutions for some parts that you mentioned. Examples:

- Auth0 for authentication

- Stripe for payments

- Billomat for billing

For other parts there are good open source frameworks, for example:

- Hasura for building an API on top of a Postgres data schema

- Frameworks like Flask, Rails or Django for building a backends

- Frameworks like React or Vue for building a frontend

- Static site generators like Hugo or Jekyll for building your website

- Hosting companies like DO or Linode and cloud providers like AWS or Microsoft Azure to host your services

There are also more tightly integrated solutions for specific niches like e-commerce (e.g. Shopify), if you build a SaaS solution you will probably have too many aspects that are unique about your idea to use such a highly standardized solution.