When you say “downloading”, do you mean adding a new plugin from the WP plugins site, or literaly downloading it from a site to your computer. The plugin has to be moved into the WP folder /wp-content/plugins.
If you have a plugin zip file on your computer you can upload it within WP by going to the plugin page,
clicking AddNew at the top
click upload to the right of search (at the top)
and finding the zip file on your computer.
If the plugin has been uploaded to the correct folder it will show up on the plugins page after that has been refreshed. If you don’t see it in plugins most likely its not been uploaded to the correct folder.
Its also possible if you didn’t get the complete plugin file, for example, WP may not be seeing it as its intended to be seen as ots not all there. Solution to that is download again
