How to ACTUALLY avoid Steam Updating your game:

Many seem to know the recommendation:

just launch through SKSE and disable Automatic Updates in Steam

This is NOT a permanent solution!

The Permanent Solution:

1. In Steam, right-click on Skyrim SE and select properties
2. In the New Window, select Installed Files on the left
3. In the Top right of the Window click on Browse
4. Now File Explorer should be open move up two folders, you should be in a folder called "steamapps" now
5. Look for a file called "appmanifest_489830" right-click on that file and select properties
6. In the New Window, at the bottom above the OK button there should be Attributes: and two check boxes, checkmark the box next to read-only.
7. DONE!

Steam will now be unable to update Skyrim SE ever again, until you go back to that file and remove the checkmark in the read only check box.