Concept Confused : Cloud Interaction with Wordpress in GCP and APP with Spring

I'm quite new to wordpress and GCP, learning programming less than one year.

Now I'm writing a project using wordpress in GCP and mobile app in the frontend and MySQL in GCP and Spring Boot as server and controller.

In GCP, Wordpress, MySQL, Apache, etc are included and managed.

[my concept architecture] [1]:

As you can see in the architecture I assume they will work, there is no need to send Rest API to spring boot and react to DB. I would like to know if I am right about the basic concept.

There is shared information in wordpress and app, for example, membership data. In app, we send request to spring and spring reacts with DB in GCP, as wordpress acquires data from DB again, a new member info is loaded automatically in the frontend. And in wordpress, request is sent to DB in GCP and Data is renewed. Once the app acquires data thru spring and DB, the new in is loaded.

I have the issue because the mentor said it is not usual practice for web to react directly with DB, and the co-workers insist establishing a connection between web and spring. Why?

The initial concept is to create a web and app that shares same database, but create different functionality, web for selling, app for interactive activities. We learned the basics in java, spring and web, but we are really unfamiliar with network concept. (The frontend part and DB establishment are finished. We are working on the Server part.)

If there is any clarification or any suggestion to modify the whole procedure, it's well appreciated. If there is any nothing I did not explain so well, I will try harder.