Spring Boot + Angular is a popular choice.
But you can choose any backend framework like Spring Boot, Django, NodeJS, etc. Based on your language of choice: Java, Python, JavaScript. Once you get a good hold of the basics, you can switch to other frameworks. These frameworks will help you hit the ground up and running in a short period of time with minimal effort so literally anyone can learn the basics of backend development
Pick any framework of your choice and build a basic REST API for CRUD operations. Later on you can add a database to persist the data, secure the access of APIs using JWT, add API documentation using Swagger UI, integrate third party APIs for SMS, News, Payment etc.
You can also explore deployment of your APIs and database to any cloud service like AWS and build CI/CD pipelines using GitHub Actions etc