The Muslim conquests in the 7th century are extremely confusing

I know the Romans and Persians were both exhausted from 20 years of war but how did they get pushed in so quickly by the Muslims?

It seems like there was very little defense and very little appetite for reconquest? Did the Egyptians and Levantines just not care that they were no longer Roman? I don’t really get it. They were all Roman citizens.

Is there any truth to the idea that Islam wasn’t really Islam until well into the 8th century so it didn’t seem so foreign? What about the idea the Arab mercenaries were left standing around after the last war between the Romans and Persians and were the bulk of the Islamic force?