The Layout Builder is the area where you can make a layout for your Products, Blog posts, Ads, Or your Advanced Gallery. Here is how it works.
Your modules have "Fields". In your E-commerce module you have product fields like Description, Price, Title, etc...
Your Blog Module is the only module where you can't add and remove fields, but all other modules can add and remove fields. Having a field and displaying it are 2 different things.
Fields are added in the module and data is added to these fields by adding/editing your Products/Items.
A Layout Builder is how the Page/Frame displays your Products/Items.
Each Layout builder works the same. You add the fields to the layout where you want to display them. You can move them around by dragging them where you want them to be displayed. And the content of the fields is edited in the module itself when editing an item.
See these individual Answers for each Module: