Coming back to Coding - how to learn to develop Gen AI

I was a great developer (C++, Visual Studio, Unix and other open systems). For the last 12 years or so, I have been in managerial and business facing roles (often in legacy industries) and now I am laid off finding the world having changed in Tech.

I am starting up on my own and finding that hiring an Engineer as a startup is practically impossible + the work is conceptually easy to do.

But, what should I do get back a primer on Generative AI? Particularly RAG of PDF files. I aim to build a basic chatbot that can answer queries from car mechanics on repairing a specific vehicle. I will feel it PDF files of shop repair manuals for that model.

What are some best practices on learning how to do this?