Fighter A can differ from Fighter B by:
1) Species
2) Background
3) the weapons they use in melee and ranged combat
4) the skills they are proficient in.
5) their ability scores.
6) how often they use Second Wind, Action Surge, Indomitable
7) whether they pick an ASI or a feat at 4th, 8th, 12th and so forth.
8) which archetype they pick up at 3rd level.
9) their fighting style
10) how they are role-played.
There a lot of things that can set the two Fighters apart.

You just need to be creative about it all.