What do you mean...
'whenever Steam opens'?
That's just shader cache.

EDIT:
Oh, you mean the file validation?

EDIT2:
What issues are you having?

Set the game's appmanifest to read only, install the script extender, only launch the game with the script extender. Game will never update again.

Also clear your shader cache and the Steam cache because your computer is bugged if Fallout 4 is trying to update every time you start Steam. Fallout 4 has not had a update in many months.

Your post is 2 days too early for the panic on the 18th man.

... You leave very little information for such passionate hatred, maybe try explaining in careful detail what *exactly* you believe your computer is doing with the game?

** make sure you have steam ON before running the game. NEVER launch the game's exe either raw or by F4SE / your mod loader, unless Steam is RUNNING FIRST.

** check your mods. Period. All of them. Make sure that they are correct for the version of the game you're trying to run. This includes things like MCM, SMM, and even F4SE itself. Don't rely on any other resource but YOU downloading after YOU read whether the mod is updated and must be reinstalled, or doesn't need to be updated.

** if you're trying to use the high def texture pack on a mid to potato machine, remove it and stop downloading it.