Personally for me a number 8 is a box to box central midfielder. A player that has a licence to get forward but isn’t devolved of all responsibility defensively.
They need to be able to pick the ball up from the back four and drive it forward either with the ball at their feet or dictating the pace of the game by keeping the ball moving quickly.
They also need to track back and basically run themselves into the ground every game.
For England it’s a Lampard or a Gerrard, which was why they couldn’t partner together in a midfield two as one needed to be a 6 whilst the other was an 8…
In a midfield three you have the opportunity to play two 6’s and an 8, or even a 10 and no 8…or you can play one 6 and two 8’s or a 10, if you’re wingers are expected to be offering support to their full backs defensively rather than just wide forwards…