About Installation Manual
Install manually extensions (components, modules, plugins and templates) in Joomla 5 is basically going to System >> Install >> Extensions and drag all the extensions you want to install to the box. It is used to install extensions and templates on existing sites or on updates that are not possible using Joomla Update Manager.
The extension packages must be downloaded from the download section of the site where you bought the template or extension, in some cases you may need to unzip a zip package that contains the extensions.
Installing the template independently is usually done when you want to use the template on an existing site with content or to update when it is not possible to do so from the Joomla update manager.
Manual installation of extensions such as components, modules and plugins usually is done on existing sites or to update paid extensions.