The easiest way is to copy a video link and paste it via the Video content block. Click on the Change video button and paste the YouTube video link. The video will be scaled automatically to fit the placeholder.
You can also insert YouTube link into YouTube embed block. Click More Button, Embed block, drag and drop YouTube embed block to a course page and insert YouTube link:
If you want to have more control over the video (e.g., adjust the iframe height and width, start at a certain point, etc), you can embed it via the HTML content block.
To do so, open a YouTube video, click on Share > Embed, and copy the embed code.
Then, paste the embed code into the HTML block and adjust the attributes.
