Unlike the old days, though, you have to actively not download data, as opposed to not actively download data. For example, make sure synching to the cloud is off (moving a folder, especially if you’re not always connected can easily be an accidental replias of 10GB), making sure an OS update won’t be downloaded (easy on windows but not easy on MacOS except in the case of iPhone tethering), and be careful about browsing (Reddit or news sites can easily use hundreds of MB in a few minutes).
Little Snitch (Mac) can be very helpful in these situations. When I'm on the road and using my limited phone data for internet, I turn on the 'limited' profile where Little Snitch doesn't allow most internet traffic (with some exceptions).
