This is a very cost-effective lecture because it is free. You can immediately make the authentication/authorization function using JWT tokens work. Due to the limitations of the lecture time (free + short 43 minutes), detailed principles were omitted, a method of implementing quickly by copying and pasting was used, and implementations that are difficult to use in practice were used (such as a technique of directly sending a JPA entity as a response, but attaching @JsonIgnore). I think this is inevitable. To understand the detailed operating principles of the technologies used in the lecture, you need to study Spring Web, JPA, and Spring Security separately.