Guardiola managed Barcelona in his first job as a manager. Look how successful he turned out.
I feel like he's the exception from the rule. Concerning his step up from Barcelona B to A, he's had it relatively easy. He is a great tactician and pretty creative. But I know he made some bold decisions but other coaches have promoted youth players and got rid of ageing/underperforming stars, too. He's had the massive advantage of not only being a great managerial talent, but to have the world's best squad at his disposal which a) was able to mostly realize his ideas and b) allowed him through sheer quality to adapt, learn and make mistakes without instantly dropping down the table. The success at Barcelona put him straight into the top bracket of managers.
So there's a seldom combination of a great talent as manager and a squad that allows him to learn and grow while still performing.
Same goes for Zidane, although I find it hard to judge him. He's clearly left his mark on football with the back to back CL wins, but although I like Zidane and dislike Guardiola, I have to admit there's not the same level of tactical acumen and creativity to be seen with him, as with the Catalan, and I'm eager to see what happens if Zidane has to go at Madrid.
These two are the only managers who went straight from a B team to the first team and instantly had great, critical success - and what really is the common denominator here is that both had the world's best teams at their disposal.
Under this special circumstance, how do we evaluate what's squad and what's manager? Upon that evalution, can we infer that someone like Arteta or any other B to A team manager will have the same success or chances at success at any given team?
Concerning the question of being able to manage a big club, how do we relate other new managers work in comparison to Guardiola and Zidane? Has Nagelsmann not won the CL last year because he's worse than these two or because he's not coaching Real or Barca? Would Guardiola have won the CL with Hoffenheim?
What I'm kinda trying to say is...Zidane and especially Guardiola shouldn't be taken as examples when infering from their success to other rookie manager's possible abilities, because all factors in their special cases considered, they're still exceptional managers, but their straight success stories are anomalies which shouldn't be taken into account.