Why is the bVII chord said to be borrowed from the parallel minor?

First, the bVII (Eb in your example) is a dominant seventh chord in most cases. It is not seldomly preceeded by a iv7 (Bbm7) chord to make it a ii-V progression. In that combination, the modes of F mixolydian and dorian are ruled out. This leaves us with the parallel minor key (i.e. "natural minor" or "aeolian").

1. What is bVII chord?
2. What is a parallel minor chord?
3. How do you use a bVII chord?

What is bVII chord?

Simply that the bVII7 chord is nothing more than a Dominant chord (7) that is built on the bVII degree of the key (Flat Seven aka the Subtonic). ... In a Major key, bVII is just a half-step below vii. If we are in the key of C, the bVII7 chord is Bb7.

What is a parallel minor chord?

G major and E minor are called parallel keys. The G major chord and the E minor chord in the key of C major are called parallel chords in the Riemann system. ... Thus, in a major key, where the dominant is a major chord, the dominant parallel will be the minor chord a minor third below the dominant.

How do you use a bVII chord?

3) The bVII on its way to bIII: When used this way, the bVII takes on the function of what is called a secondary dominant chord. It briefly makes the bIII sound like a tonic chord: C Bb Eb G.
...
2) The bVII on its way to bVI: This makes a great descending-bass-line progression:

1. C Bb Ab G.
2. C Bb Ab Bb.
3. C G Bb Ab (G)