Hi Toxic!
Basically, a shift bid works off of seniority and metrics. There's, say, 5 different shifts available with varying days off. The most senior agents with best metrics will usually get first pick, and tend to pick the best schedules. Newbies in general at most jobs seem to get stuck with either mediocre or downright undesirable shifts, but only for a while until the next shift bid. I know at my current job (Dish) you could swap shifts with another newbie while still in training if you both were okay with it.
As for the questions asked, just some general tech support stuff (doesn't have to be Apple based, use general tech knowledge here) and a roleplay call. Don't worry about knowing the answers; the same here holds true for the third interview. They are more concerned in how well you take care of the customer, so even if you don't know what you are answering is correct, say it with confidence and say it in a pleasurable tone. Basically, you can BS your way through the call but if you do it well, they don't care! They can teach the tech stuff; they can't teach good customer service skills. They are more worried if you have those, than tech knowledge. Good luck!!!
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Stat, just make sure you know how to interact with people well. You don't need a ton of tech know how- just know what RAM is, what an OS is.... basic basic techno jargon you might hear if you were to call in for support. They are far more interested in seeing if you can keep a confident, pleasant tone throughout the call and handle the roleplay situation (fairly short and sweet) well. Good luck!!