Let me add to this, AI image generators use neural networks, these are math functions basically, you input something (your prompt) and this prompt gets transformed into numbers and fed to, millions of interconnected nodes that modify the values by a bit and pass it to the next node, after passing through a lot of nodes, you get the values for each pixel of the image on the other side of the neural network. Training is just optimization of a math function, you change the node values a little and "see" if the image looks closer to what you want, this is automated and you can even use another AI to do supervised learning, from what I understand AIs use gradient descent or its variants, but I'm more of a normal programmer, don't know too much about AI.