How and when did 1 #Bitcoin become 100,000,000 sats? How was it decided? And by whom?
Discussion
Satoshi did it. He recognized that the unit was arbitrary and the smallest unit could be subdivided as necessary. I think some of that is in the white paper.
Source code
What was that unit referred to as in the code? And if there isn’t a name for it in the code, who suggested using “Satoshis” or “sats”?
Read the white paper and Banking on Bitcoin is a good documentary on YouTube. Keep in mind it is old and much has changed such as the introduction of the lightning network. The Book of Satoshi has almost all the email correspondence with Satoshi. https://bitcoin.org/en/bitcoin-paper
This article explains some of the maths behind Bitcoin’s 21 Million supply cap which isn’t directly your question but it’s very much related.