Not sure I can give an ELI5 explanation, but for me this the closest to what you are after in two short videos on the topic. At least they helped me get up to speed fast.

Hope they will do the same for you ;-)

Large Language Models from scratch https://www.youtube.com/watch?v=lnA9DMvHtfI

Large Language Models: Part 2 https://www.youtube.com/watch?v=YDiSFS-yHwk