You need to use proper flows to communicate between the applications.
If you are requesting a data from a server-side application using httpclient (like winforms, mvc, razor) to another server-side application, you can (should) use the Client Credentials flow.
You can create a hosting application (that has a Program.cs) module and add the Identity Module layers following the module architecture. If you want to host the API of the identity module as well, you can add the HttpApi layer of the Identity module. If you want to host the UI of the identity module as well, you can add the Web layer of the Identity module.
There are different architectures based on your needs if you understand the module architecture.
You can check the source code of eShopOnAbp Identity microservice that hosts Identity Module.
Keep in mind, complications and complexities increase when you move from monolith to distributed systems.
Can you create an issue at https://github.com/abpframework/abp/issues with the tag community-article-request? That can reach a wider audience.
You can start generating your modules by using the module template. Each module has its host options so your development teams will be able to test their module individually.
The Plugin Modules is about the end application.
As a sample, the module building teams can publish their module (the dll file) to a remote file system and the end application can poll and get these dlls to a local file system where the end application runs. Your end application will be configuring Plugin sources by pointing out that local file directory which will have the dlls.
... is it possible to show a list of books on the Author page? (So by picking an author on the Book modal, the book would automatically be added to the book list on the author's modal).
I couldn't understand it clearly. Do you want list of books inside the author create/update modal or list of authors inside the book create/update modal?
If you want to list the books in author create/update modal, it will be very ineffective since there should be way too many book records than the author records. When creating a new author, you will be loading all the books just to assign to an author.
If you want to do so, use the BookAppService to get all the books and you can use select2 library to add single or multiple items.
If you want to show the author list inside the book create/update modal, it is how the tutorial approaches.
Can you check the response headers? If you are logging as a tenant user, response should have tenant information.
Can you share the information about your solution like
On which page do you receive this error?
As I have mentioned in this answer, you can not use a token of an application in different applications. This is not related to ABP or IdentityServer.
You need to register your applications (Frontend web client. backend web client, wpf client, windows forms client) as clients with correct redirect uris and correct authorization flows (like authorization code flow).
Can you share steps to reproduce?
Try adding * to the WebOrigin of your realm in keycloak configuration. Or restrict it with your swagger endpoint.