https://answersingenesis.org/bible-characters/who-were-the-nephilim/
Here are 4 different views:
Here is a table with a brief summary of the four popular views discussed:
Name View in short
Fallen angels view Satan and/or his fallen angels bred with human women and had offspring that were called Nephilim.
Fallen angels overtook men view Fallen angels and/or Satan possessed men and caused them to breed with women.
Sethite view The sons of God were the godly line from Adam to Seth down to Noah, and the Nephilim were fallen children who sought after false gods.
Fallen men view Godly men (sons of God) took ungodly wives, and their descendants (Nephilim) followed after the false gods, rejected God, and fell far from God in wickedness.