For partial deployment projects which are not Request All Files, you can view and manage the list of files in the package for the release using the Manage Files link.
The list of files can be managed here (adding, removing, and sequencing). New builds referencing the release will have these files in the package by default. However, the typical model is for developers to reference the release during build, and add files to the package. The release package will then be updated to add the files. It can be managed in either place, however, this is typically considered a more technical activity managed by the development team. Release Managers who have update access to the Release screens do not typically want to get involved with file-level detail.