Go to your admin page of the Joomla and click on extensions and choose install/uninstall and there you can install your extensions. First you need to know what is extension, there are three type of extensions, one is called component, second is module and third is plug-in. components are like stand alone or application, like anything from video gallery, forms etc. module is for like add-ons to your website like menu, banners etc. And plug-in is like a thing which can run in background like to embed videos in your website. So to download extensions, go the website called extensions.joomla.org and there you can see different types of extensions there are some paid extensions but many of them are free so browse it and choose whatever you want and then download it to your computer and then upload it to your Joomla.
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