Open your Google Docs document
Click where you want the video link or image to appear
Go to Google Drive and upload the video if it is not already there
Open the video in Google Drive
Click the three dots or right-click the video
Select Copy link
Return to Google Docs
Paste the link into the document
Optional: Insert a clickable thumbnail by adding an image and linking it to the video
Optional: Use Insert > Drawing > New to create a linked video image
Save the document