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.
Now go to your TYPO3 version 9 website and create a new element on the page where you want the video. Select 'Responsive video' under the tab 'Content elements' (8.2.3)
Below the tab 'Video', click 'Add media by URL' to open the fields to add the address for the Youtube video (8.2.4)
Paste your copied URL in the text field and click add (8.2.5)
After adding the address, the video is presented in the element. Here you can handle the file: delete, hide or move (1), add a unique title (2) and add a description for the video (3) - (8.2.6)
Under the inserted video, you can also see some additional settings (8.2.7), which might be also useful for use: