The easiest way is to copy a video link and paste it via the Media content block. Click on Change video button and paste the YouTube video link. The video will be scaled automatically to fit the placeholder.
If you want to have more control over the video (e.g., adjust the iframe height and width, start at a certain point or enable the autoplay), you can embed it via the HTML content block.
Open a YouTube video, click on Share > Embed, and copy the embed code.
Paste the embed code into the HTML block and adjust the attributes.
To enable the autoplay, add the following attribute ?autoplay=1 to the video embed code, as you can see in this example:
<iframe width="100%" height="500" src="https://www.youtube.com/embed/t_BBStwl1Nc?autoplay=1" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>