When a MainWP Extension requires a 3rd party to be installed on Child Sites, the Dashboard will prompt you to install the required plugins.
The required plugin check is triggered when a page of an extension is opened (e.g., Dashboard > Extensions > UpdraftPlus)
The popup modal offers the possibility to install the required plugins with the Install Plugin button, or to dismiss it by clicking the Close button.
The installed plugins will automatically get activated on child sites. The Dashboard needs to be Synchronized with the child sites for the extension to be able to see newly installed 3rd party plugins.
And if the modal was dismissed, the next check for required plugins will take place in no less than 30 days.