You can install a plugin or theme from the WordPress.org repository across multiple sites at once.
Go to Manage > Sites.
Select the sites where you want the plugin or theme installed.
Click the Install dropdown button in the action bar.
Choose Install Plugins or Install Themes.
In the modal that appears:
Type at least 3 characters in the search field to find plugins or themes from WordPress.org.
Select the one you want from the results.
Optionally, toggle Activate to automatically activate it after installation.
Confirm the action.
The installation will run in the background. Check the Tasks page to track progress.
Tip: You need to select your sites first — the action bar with the Install button only appears when sites are selected.