H1B is a working visa, you only need I797, the approval notice for H1B to work.
EAD is associated with F1 OPT or filing of I485 (for green card). The F1 OPT EAD is one year and you can set your H1B start date the next day of F1 OPT expiration date. And for H1b you have 6 yrs with possible extension if you filed i140 and I485.
I485 EAD has nothing to do with H1B. But if you use EAD from I485, you lost your H1B status right away.