Hagerty's Kyle Smith walks you through how to properly bend brake or fuel lines in the first video, and in the second video, how to properly create an SAE/double/inverted flare on your brake and fuel lines for your classic car.
That's a good question! Apparently there are SAE and Metric nuts but the lines themselves are probably universal.
The kit I have has both metric and SAE fittings, although since I no longer live in the rust belt will probably have the newer cars traded off before they will EVER need brake lines so the metric fittings have just gathered dust.