How did Jimmy end up working there? and What is the significance of those scenes?
Towards the end of Breaking Bad, after Hank is killed.
Walt’s involvement in the drug business is exposed, and as a consequence, so is Saul’s. We see Saul and Walt in the basement of the vacuum-repair shop, as their new identities are being created to allow them to escape jail.
As Saul remarks to Walt, the best he can hope for is to be manager of a Cinnabon in Omaha. True to his word, he achieves this glittering dream*, which is what we see in the black-and-white season-opening scenes in Better Call Saul — our lead character, after the events of both shows, living his life as best he can.
As of the season 5 opening scene, we learn that someone has recognised Saul from his Albuquerque days, putting him in danger of being found by the police. He calls the vacuum-repair shop to get yet another new identity set up, but then changes his mind, and says he’ll handle the situation himself.
It seems likely that the last episode or two** of Better Call Saul will show us what Fate, Cinnabon, and Omaha have in store for Gene/Saul/Jimmy.
* We’re not shown how, but from everything else we know about Jimmy, I figure good old-fashioned elbow grease with a sprinkling of expedient lying.
** Turns out it needed the last four!