You can add a Youtube video on your website, by copying the URL of the video on the Youtube site, and then adding it to the 'Responsive video' element in TYPO3. This will create a video player as shown below. This type of element will allow the video to automatically adjust to different screen sizes (8.2.1)
This element can also be used for video files uploaded in the Filelist.
Go to the Youtube website and find the video you wish to use. Copy the URL address (8.2.2)
Paste your copied URL in the text field and click add (8.2.5)
Under the inserted video, you can also see some additional settings (8.2.7), which might be also useful for use: