When an artifact is requested from a virtual repository, the order in which repositories are searched or resolved is local repositories first, then remote repository caches, and finally remote repositories themselves.
Within each of these, the order by which repositories are queried is determined by the order in which they are listed in the configuration, as described in General Resolution Order.
For a virtual repository, you can see the effective search and resolution order in the Included Repositories list view in the Basic settings tab. This is particularly helpful when nesting virtual repositories. For more details, see Configure a Virtual Repository.