On click on submit button, I want to check that field have some value.
if the field does not have value then do not save the detail

I'm struggling to understand how to pull triggers and a apex classes FROM a sandbox using visual studio code / salesforce dx. I'd like to pull triggers down from an org to edit them in a batch but can't find the method to do that. I've looked around at documentation but I've struggled to find anything that would allow me to do this specifically.