Genesis 6:4 There were giants, (Nephilim), on the earth in those days, and also afterward, when the sons of God came in to the daughters of men and they bore children to them. Those were the mighty men who were of old, men of renown.
Giants <05303>. נפילים or נפלים Nephilim; from 05307 (to cast down); properly, a feller, i.e. a bully or tyrant: — name of two peoples, one before the flood and one after the flood: —
Numbers 13:33 "There we saw the giants <05303> (the descendants of Anak came from the giants <05303>); and we were like grasshoppers in our own sight, and so we were in their sight."
Firstly, there are no fallen angels. Satan is not a fallen angel.
Satan is the Hebrew word for adversary and is so translated in the King James Version in 7 places.
A ‘satan’ can be a good or bad adversary.
‘Satan’ and ‘Devil’ in the New Testament are used as a personification of the proclivity to sin within man, both individually and collectively.
Temptation to sin comes from within man, not from a supernatural tempter.
Angels cannot sin; they do God’s will perfectly.
The faithful are promised equality with the angels.
If angels can sin and die the promise of eternal life would be a sham.
In the place where we are promised equality with angels, we are also told that angels do not marry – they do not reproduce, because they live forever.
Matthew 22:30 For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven.
Mark 12:25 For when they shall rise from the dead, they neither marry, nor are given in marriage; but are as the angels which are in heaven.
Luke 20:35 But they which shall be accounted worthy to obtain that world, and the resurrection from the dead, neither marry, nor are given in marriage:
The Lord’s prayer makes the request that God’s will be done on earth as it is done in heaven – i.e. perfectly.
The ‘sons of God’ is a name given to those who are led by the spirit of God.
Romans 8:14 For as many as are led by the Spirit of God, they are the sons of God.
It can refer to angels or men.
As angels do not marry, the ‘sons of God’ in Genesis 6:4 must refer to Seth and likeminded men who called upon God and were therefore ‘led by the spirit of God’.
Genesis 4:26 And as for Seth, to him also a son was born; and he named him Enosh. Then men began to call on the name of the LORD.
The ‘daughters of men’ refers to the daughters of Cain and likeminded men who rejected God and lived by their own desires.
The text does not say that when godly men married godless women, their male offspring were ‘Nephilim’. The use of the word ‘also’ indicates that their male offspring became mighty men who made a name for themselves, in addition to the giants – the inference is that both kinds were bullies.
All those living died in the flood apart from Noah and his family, so the giants spoken of in Numbers 13:33 cannot be literal descendants of the giants in Genesis 6:4 but must be understood to mean that they suffered from the same physical abnormality. Today we know that gigantism is usually caused by a tumour on the pituitary gland of the brain. In some cases the condition can be passed on genetically through a mutated gene.
Demons are a different thing entirely.
In the New Testament demons are defined as ‘unclean spirits’.
Spirit is the way that a man thinks – an unclean spirit is a mental illness.
May I suggest that you read Do You Believe in a Devil? https://www.thisisyourbible.com/library/show/162
And Demons - A Biblical and Historical Study https://www.thisisyourbible.com/library/show/189
I hope you find this helpful.