The difference is basically the wing. The 777x wing is significantly larger than the 777.
I have flown on a 777 several times. Very, very nice flight. No complaints. Have not flown on a 787...yet.
Full disclosure: 32-year Boeing (retired) electrical engineer.
Thanks. Is the 777x similar in concept to the 737 Max? Meaning, they tried to fit larger more fuel efficient engines onto the fuselage but had to reinforce/enlarge the wings and reposition the engines to accommodate?