Izanagi is a variation of the jutsu of Creation, which needed both Yin and Yang powers to create things.
But, you must remember every living thing has a Yin (a mind) and a yang (a body). So, the reason of why the fodder Uchiha could use Izanagi, it is because they had at least a little yang part that could be sacrificed to execute the legendary technique, and this yang was the life in the Sharingan they sacrifice.
Danzou in the other hand could use it without their sharingan losing life because he had the Senju DNA.
Your doubt:
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The background that explains the origin of this jutsu:
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Summing it up: The Senju DNA is used only to reduce the drawback of the technique. That's all.