Or, you can copy and paste code from examples, StackExchange, open source code, etc. Or you can read about it once, use it, and forget why it worked.
Besides all that, though, it's really the fact that LLMs bring up interesting ways to tackle problems that I hadn't thought of before, or uncover neat libraries/packages (when I program in R) that I just am not aware of.
If you write it by hand you don't need to "learn it thoroughly", you wrote it
There is no way you understand code between by reading it than by creating it. Creating it is how you prove you understand it!