This is a meta issue to group together issues for various inconsistencise in the Appearance and Extend sections in administration as to how Drupal refers to and provides navigation for bringing contributed modules and themes into the file system. I've assigned this to the user interface text component, as this mostly seems related to what this action is called. However, there is at least one path anomaly as well.
Where I have filed issues on this topic, I am trying to keep the wording to address one particular independent inconsistency. However, if a new wording is agreed upon by the community, it will percolate into the other issues and potetially cause git merge issues between one Drupal issue and another. Thus this meta issue.
The most active discussion on what the terminology ought to be seems to be on #2577407: Module install form has two "install" buttons that do different things, which appears to be a duplicate of #1207354: Install new module v/s Add new module. It covers modules/Extend. However, themes/Appearance have a parallel structure and I believe it would be best if any terminology changes are harmonized across modules and themes.
I've added the following issues which spring from or will be impacted by the resolution to this issue:
#2888652: Move instruction and button for loading contributed modules or themes next to each other
#2888654: Loading a theme into the file system has an inconsistent path
