The Dev Portal uses the Swagger Aggregator to make all your OpenAPI specifications, even the ones spread across different projects, available in a unique place.
As mentioned here, the version of your api-portal needs to be at least 1.14 in order to configure the api documentation correctly.
Merge the APIs of multiple projects
To include specifications from different projects, you can import them by URL using the Advanced section of the Design Area of the Console.
Check the Swagger Aggregator Advanced Config for more details.
Remember to set appropriate request and limits levels to the Swagger Aggregator microservice in order to avoid performance issues.
Check out the dedicated section for more information regarding requests and limits.