If you are a software engineer who wants to stop treating AI as a black-box API, this is the place to start.
Andrej Karpathy (former Director of AI at Tesla, founding member of OpenAI) has an open-source repository and an accompanying YouTube lecture called "Let's build GPT: from scratch, in code, spelled out."
Instead of burying you in math formulas, he builds a Transformer in PyTorch starting from a blank file, explaining every single tensor shape and design decision along the way.
It is, without exaggeration, the single highest ROI piece of educational content for backend engineers moving into AI.