The Federation (and Starfleet, for that matter) is now a shadow of its former self, while technology has advanced pretty far beyond what Discovery’s crew is used to, and the state of relations between the wide variety of races and lifeforms is now a mystery they’ll have to catch up on very quickly to do their jobs.
Discovery, which began as a prequel to series to Star Trek: The Original Series, has now gone way beyond Nog’s time in Starfleet.
At the end of the previous season, the crew had to jump about a 1000 years into the future, and Season 3 has shown that things are a lot different in the galaxy.