When you post a YouTube video, you can embed the video in the description so that it displays as a playable video rather than just a link.

Go to the YouTube Studio (https://studio.youtube.com/).
Sign in to your YouTube account.
Access the Video Manager:

Click on "Videos" in the left sidebar to access your Video Manager.
Edit Video:

Find the video for which you want to add an embedded video link and click the pencil/edit icon to edit the video.
Description Section:

Scroll down to the "Description" section of the video editor.
Insert Video Link:

In the description box, paste the link to the video on its own line.
Autogenerate Thumbnails:

YouTube will automatically generate thumbnails for the video links in the description. Wait a moment for the thumbnails to appear.
Embed the Video:

After the thumbnails appear, hover over the video link in the description, and you should see a "pencil" icon to the right.
Click the Pencil Icon:

Click the "pencil" icon, and YouTube will give you the option to embed the video. Click "Embed video."
Save Changes:

After clicking "Embed video," save your changes.