How to get focaccia with a larger crumb?

You can start your focaccia efforts using a poolish, a biga, or a regular sourdough starter- as they are all versions of pre-ferments, and are places to start bread dough building. Large crumb breads typically require a high hydration dough, meaning a very wet, sticky one. You also need strong gluten formation, which is what is accomplished with using that preferment, and all the folding done in the rising that follows. While you probably won't be able to pull off a professional bakery focaccia without a lot of practice, you can certainly produce one that is just as tasty. Just pick a recipe from a reliable source and give it a go. Doesn't matter what they call the starter- if they call it a biga or poolish, or simply a starter. While there are differences between them, for most home bakers, it isn't worth fretting about. As you gain experience with your starter of choice and dough, you will be better able to tweak things to suit yourself. In the meantime, it will still be pretty tasty bread.