Edit: Thanks for the explanation and all the comments. I had no idea of how it was and also wasn’t expecting it to be like that. Curiously interesting!
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There are a couple ways/reasons that a hive would create a queen. Ideally from the hives point of view, it is spring/summer and the hive is ready to swarm and create a second hive. The worker bees will create a special cell for the current queen to lay an egg in. This cell is going to be facing more vertically as opposed to horizontal like most cells. After being laid, this egg will be fed something called royal jelly. This is the real trigger. It is primarily diet that causes what would have been a worker bee to become a queen bee, as genetically they are not different. After being fed this diet, it will take about 15 days to go from fresh egg to emerging queen (a worker is about 21 days)
Worst case scenario is that the current queen dies unexpectedly. If this happens, the workers will take some of the youngest eggs they have, convert the cell so that it is vertically facing, and begin to feed the egg/larvae royal jelly. Everything else is the same, however the success rate is somewhat lower depending on the age of the egg when the conversion begins.
TL:DR It is primarily diet that causes the difference. There are other changes as well with development, but all are triggered by diet
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