[Discussion] How were the Bijuus created?

[KCMNaruto]

Off topic, but how was BZ "born" of kaguya's will got sealed away?

Pretty simple. Black Zetsu isn't full Kaguya's will but just part of her will taking on form by Kaguya utilising COAT.

Also it happened at the end fight against Juubi. Black Zetsu didn't talk about his creation after Hagoromo was dying but only when Hagoromo and Hamura sealed Juubi in Hagoromo, unless BZ somehow got created by Kaguya using Hagoromo's weakness and strugglign being on verge of dying while separating Juubi's body from Juubi's chakra, however Hagoromo didn't notice anything.

Well something like that.

That is why its more logically correct if this is what happen: 1. Hamura and Hagoromo seals the Juubi's shell into the core of the moon. 2. Hamura and Hagoromo seals the Juubi's chakra into Hagoromo. 3. Hamura dies and Black Zetsu is born from the black core of the Moon. 4. Hagoromo unseals the Chakra from himself as gives it life.

NO it is in fact stupid of you to claim that Moon was created before Hagoromo was on verge of dying.
Black Zetsu couldn't be born of the Moon in any way or shape.
Hagoromo had to use COAT having yin and Yang to extract tailed beasts.

Having just Ten tails chakra could not make Hagoromo Juubi Jin which goes against the manga.

The most fallacy in your theory is that Tailed beasts disagree and Kurama stated Sage created them using COAT then sealed Empty Ten tails's husk in Moon.