Java User Questionssentiment_satisfied

So I am a bedrock player and I recently switched to Java Edition to try out some mods. I've gotten good at Java, but I still have some questions;

How do I customise a shield? Do I use a command to make a banner pattern print on it? And how?

I have a few more questions but I'm lazy so feel free to just tell me any tips and commands you have. Tysm if you do,

                        -a Newly Java-ed Player