The most noticeable difference is the totally revamped administrator, which is updated to a modern design with many simpler and more friendly user-interfaces. Also, starting with Joomla! 3.0, Joomla! is now device responsive.
Install Joomla Components, Modules, etc.
Download any extension from joomla official website and make sure you download the complete package.
Now go your joomla administrator area => Extensions => Install/Uninstall.
Select the .zip format package from your local drive and click on upload & install button.
Now you get the installation successful message.