When you need to add a media asset to a scene, you can use any of the assets that you used in other videos or that you uploaded independently. These assets are saved in your media library, where you can search for them easily.
In the scene, hover over the media placeholder to see the blue borders and then click inside the placeholder.
In the right panel, click the checkered thumbnail.
Your media library is displayed, with all the media assets that have been uploaded to your account until now:
Search for the media asset using one or both of the following options:
In the example below, searching for an asset that has the word card in its name retrieves two results:
When you find the media asset that you want for your scene, select the asset so that it is highlighted in blue and then click Use at the bottom of the page.
The media asset is added to the scene: