I watched this video. there is a question.
The demo song is in the key of G
But MicroChords reuse Dm, no Dm in G key
Of course, I also noticed that the chord of Bar is C
So enter MicroChords according to the chord of the BAR it is in?

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I work in MIDI and I have found that it doesn't matter what chord you use, see below:

I still wish there were rules.
Otherwise, you need to keep trying to find your own satisfaction.

I agree.
My question is why do we have to put a chord in first when working with micro chords? Especially when it doesn't matter what chord you use!

There are rules, but fortunately they are for guidance, not mandatory.

A couple of sayings:

"Rules are there to be broken".

"Rules are for the obeyance of fools and the guidance of wise men".

Music uses "tension" and "release" to create colour and drama. Out of key chords are often used precisely to create extra tension, extra surprise. Quite often a chord can be "borrowed" from a parallel scale/mode to create an unexpected movement, or it can be an "approach" chord, deliberately out of the key so that it "wants" to resolve to the key.

In the key of GMajor, there's no Eb, but there is an Eb in the Phrygian, Aeolian and Locrian modes.

Demonstrating All 7 Modes in Parallel [MODAL MUSIC THEORY]

I think that underlying chord would make a difference if you were to put exceptions in the micro chord dialogue. The instruments that have the expectation would use that underlying chord.

I'm not an expert but that makes sense to me.

Mike