Multi layer API system architecture plan

I am currently planning on creating an application.

I want this application to run on Desktop, iOS and Android. What I have done so far is as follows:

• SpringBoot API server which handles database fetching/posting. This server also handles the desktop webpages.
• Flask API server which handles certain calculations to and from the Spring server.
• The mobile apps will be made using react native.

The problem

I am going to need to handle user sessions (via a login).

Is there a single unified way of handling user sessions over all these applications?

Otherwise I am going to need 2 different SpringBoot servers. One to purely act as an unauthenticated API server to serve the main data, the other which will authenticate the user and call the other SpringBoot servers API's...

The React-native will have its own login implementation and then also call the unauthenticated SpringBoot server.

So is there a cross-platform login system that can reduce the need of having 2 Spring servers?

Thanks.