How do you figure out how to build an AI app?

Hi, I'm a software engineer interested in building an AI app. I have a degree in Computer Science so I know how to code. But I'm faced with two problems so I hope I can get some advice.

1. I have never had experience in building an app or anything outside of class projects. I'm good at solving problems when the problems are well-defined and all the resources (such as coding environment) are provided to me. I have a general idea of what product to build, but I don't know where to start. I tried to google search questions such as "what tools to use for backend or front end" but there are so many options and all the information is very scattered. I'm not sure how I can start with actually writing the code for the app.

2. I have theoretical knowledge in AI, such as what a transformer model is. But when it comes to a specific ML problem, such as building an agent that can teach blah blah, I'm stuck. There are so many open ended questions such as collecting the right data, choosing the right model to train etc. I feel there is a big gap between what I know and applying what I know to actually implement something or solve an open-ended problem

I really admire those who teach themselves how to code and build amazing apps. I don't know how they did that. For those of you who had such experience, could you share how you figure out the two problems I mentioned above? I'd really appreciate your help!