The following is not a take that will get you a job or teach you precisely how LLMs work, because you can look that up yourself. However, it may inspire you and you may create something that has a better-than-lottery-ticket chance of being an improvement over the AI status quo:

Without reading about how it's done now, just think about how you think a neural network should function. It ostensibly has input, output, and something in the middle. Maybe its input is a 64x64 pixel handwritten character, and its output is a unicode number. In between the input pixels (a 64x64 array) and the output, are a bunch of neurons. Layers of neurons. That talk to each other and learn or un-learn (are rewarded or punished).

Build that. Build a cube where one side is a pixel grid and the other side delivers a number. Decide how the neurons influence each other and how they train their weights to deliver the result at the other end. However you think it should go. Just raw code it with arrays in whatever dimensions you want and make it work; you can do it in Javascript or BASIC. link them however you want. Don't worry about performance, because you can assume that whatever marginally works can be tested on a massive scale and show "impressive" results.