So far we have only been looking at how major chords are built from notes of the major scale. Even though minor chords can be built the same way by stacking thirds from the minor scale, it's not the preferred method. We still use the major scale as the foundation for all chord types whether they be minor, seventh or any other chord type.
You could indeed say that a minor triad is built on the first, third and fifth notes of the natural minor scale but then we need to think multidimensional. It makes more sense to think all chord type's relative to the major scale.
The only difference between a major triad and a minor triad is the third scale degree, which is flattened in the minor chord. Whenever we use the terms "flat" or "flattened" we simply mean to lower a note by one semitone (half step). Likewise, when we use the terms "sharp" or "sharpen" we simply mean to raise the note by one semitone. By thinking of scale degrees in this way it makes it easy memorise some straightforward scale formulas that allow us to think of all chord types relative to the major scale.
Minor chords can be triads or extended. The formulas are easy to remember, the third is flattened in the triad, and in the extended minor chords the third and seventh are flattened. All other notes are the same as you would find in the major chords. For example, Cmin13 and Cmaj13 differ only by the third and seventh scale degrees, which are both flattened. Here are the scale formulas for minor chords.
For the sake of clarity, the following shows how the major triads and major seventh's compare to their minor counterparts in the key of C.