The New Testament quotes from an older, more accurate source of the Old Testament, which is evident based on its quotations being in harmony with the Greek Septuagint of the Old Testament and the Dead Sea Scrolls. Both of these versions of the Old Testament date to the 2nd and 1st centuries BCE. See here: https://youtu.be/2X0bmLOHR8M
Satan does refer to a particular entity/ individual in the Old Testament when its used in conjunction with the definite article (i.e. ha satan = the adversary); this being is present in the Book of Job and in Zechariah Chapter 3.
The understanding of spiritual beings apart from Yahweh and his angels evolved into believing that they included the gods of other nations, as well as Satan and other heavenly beings who were cast out of Heaven.
Luke 10:18 And he said to them, “I saw Satan fall like lightning from heaven.
1 Corinthians 10:20 No, I imply that what pagans sacrifice they offer to demons and not to God. I do not want you to be participants with demons.