Title says it all. If I bought the DLC, would it be in every save file?
As long as every save file is connected to the same Nintendo account, yes.
However, when I did this, a message saying ‘a user is already connected to this account’ at first. This lead me to assume that it wouldn’t, but when I checked next, I was able to access both the Isle of Armor and the Crown Tundra.
I hope this helped!
Unfortunately this doesn't apply to the Nintendo Online, though