In the study of formal music theory, one studies "harmony", defined as the movement of chords. What chord one uses depends heavily upon what came before and what comes after a given chord: where have you been and where are you going.
For example, if you are going to play a C major chord followed by a D minor chord, you might want to add "interest" to get from the C to the D. One way of doing that is to play C#dim7 between the C major and the D minor. It doesn't belong in the key of C, but it does belong in the key of D and is used as a "passing chord" getting from C to D.
Another factor that influences chord choice is the sound you want. If you want something to sound "sad", you can use a "related" minor chord. C major is comprised of the notes C, E and G. It's relative minor, A minor, contains two of those three notes, C and E. It is "close" to the C major, but provides a different "flavour". It is an example of chord substitution.
In your example, you have substituted a D major chord, D, F# and A for a B minor chord, B, D, F#. The B minor chord has two notes in common with the D major, sounding similar but with a different "flavour". Using a chord built on the sixth of a key - a minor chord - is a common substitution for a major chord.
There are many other ways of substituting one chord for another.
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