Open the record (in the video example, a Stakeholder has been used)
Scroll down and find the File tab in the middle section. Then click on the left hand File dropdown.
On the left, use the File dropdown and select "Link"
From the file library you can search and link for your file or upload a new file > double click to add the file to the record
The file will be listed in the table of linked files in the center window of the record
Select the view icon to open the file