Currently, to add an app to a solution, you must either manually add the app you created in the environment or select the solution and create a new app.
In a default environment, this is a natural operation, but in other (more purpose-built) environments, it is a bit of onerous task. (e.g., a dev environment for creating apps before moving them to production).
A new preview feature was added to the Admin center that would help it - "Create new canvas apps as Dataverse solution (Preview)-.
This is one of the additional features that supplement ALM, and by enabling it, newly created canvas apps become part of Solution by default.
Even though it is created from top or Apps screen.
The location to be saved is the "Default Solution", and no settings were found that would change this to any other arbitrary solution.
It is important to note that with this setting ON, makers need security roles updated for this environment to create app inside solution.
Also, in my case, It took about a day for the feature to be reflected after activation.