You should be able to post the YouTube URL directly in the body, and it will automatically embed the video. Note that you must use the full URL that looks like this
https://www.youtube.com/watch?xxxxxx
Not the short version that looks like
https://youtu.be/xxxxxx
If you have problems, let us know. The module VideoOrSocialPostEmbed is required, and it needs to be added to the body field. We can do this for you.
https://www.youtube.com/watch?v=t8Lb14kRRBc