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Variations on this question appear to be a common unanswered question; often they have a comment or two that I don't understand. I'm hoping someone can answer with some general background information, the concepts.
E.g. I see reference to "rooting". Do I need to root a phone before I can upgrade it? I've had previous Android phones that got auto-updated (either by the carrier or manufacturer: I just find a message that says: "a new update has been downloaded, click here to install"), and I didn't need to root them.
E.g. Can any smartphone be upgraded to Android 4.4? If not why not? And why might a manufacturer not want users to update their phones? Are some manufacturers more open to having phones updated, and some not? Does anyone keep a list of "upgrade-hostile" manufacturers?
E.g. does the phone being sim-locked (to a specific carrier) matter?
I'm not going to say what my phone is because that will distract from the more general answers I'm after.