Instead of saying "all of it", let me differentiate what is important to learn in your bones and what is more or less solved (or at least secondary).

You need to understand abstraction layers in the code and how to mentally navigate between them. Every change I make to code goes through a battery of concerns at different levels and perspectives: does this cause any security concerns? Are there unintended downstream effects? What is using this code I changed? How does it affect users? Are there performance concerns? Are there edge cases I am not considering? Can this be done in a cleaner way? Did I make something hard (encode logic/values) that should be soft (config setting, database value)? Are errors being caught? How are errors tracked and observed?

It is most necessary to know the right questions - an LLM can help solve them. Be excessive and wasteful with questions until you internalize when they are helpful. You will be surprised at the things you didn't consider even with small changes to an existing codebase.

You need to be able to read code and reason about it. If I was learning to code now I would spend most of my effort on reading code and conversing with an LLM to explain logic I don't understand, generate Mermaid graphs of code architecture, etc. You can rapidly level up by using LLM to help fill gaps in understanding.

Before all that, you need to know the basics: loops, data structures, variables. You need to understand tech stacks, how and why different layers are distinct (frontend, backend, database, logging, infra etc), how the communicate and how data passes through them.

In other words, architectural understanding and reducing "unknown unknowns" is the priority. If you know you don't know something, it is increasingly easy to address it directly, even if it takes a few more iterations.