The answer to all of your question is....maybe. Its going to depends on your state, county, company, perhaps division within the company, and maybe even the whims of your scheduler.
Some companies have really high standards They might require xx years of service as an EMT, additional training, perhaps even current enrollment in Medic school to be assigned to an ALS truck(or everything but medic school for the BLS 911 truck, if they even have such a thing). They might go the other way and put a first day basic on an ALS truck. I've seen both and a dozen shades of grey in between.
As the senior members here are fond of saying, if you've seen one EMS system, you've seen one EMS system. Sometimes it doesn't take more than crossing the street to another service to see a whole new way of doing things, good bad or indifferent.
BLS is primarily about human transportation. While primarily BLS 911 systems exist, most 911(in my experience, not necessarily broadly applicable) is ALS or tiered. BLS is most utilized in the commercial sector for non emergent transportation of people from place to place(home, clinic, SNF, DOB, etc). I'd be willing to bet if you added up all the ambulances in service nationwide, the majority of BLS trucks would be commercial IFT trucks. My company runs 150ish ambulances. Clip the 20 or so ALS rescue trucks, a dozen ALS transfer trucks, supervisors and such and you have about a hundred BLS trucks left standing. 7 of those are dedicated B/B city trucks. Again, I don't mean to say that my organization is representative of the world, but it's a data set to get your mind turning.
As a new basic, you will most likely doing commercial BLS. If you're organization does BLS 911, you'll promote according to the rules of that company. When you're ready, if you decide to stick with it, you'll go to medic school. Once you get out of rehab from your medic school induced drug and alcohol addiction, you'll go to work as a medic in the capacity available to you at that time and place. No medic(or basic for that matter) here has walked exactly the same path from there to here, so it's very hard for us lay you out a path.