The devil and Satan are a New Testament personification of the propensity to sin within man, both individually and collectively.
The doctrine that the devil is a ‘fallen angel’ is false.
No such entity exists.
May I suggest that you read “Do You Believe in a Devil” here:
https://www.thisisyourbible.com/library/show/162
I hope you find this helpful.
God bless,
Glenn