Acquia Cohesion renames to Site Studio from version 6.2. Tell me more.
Yes, the component link field supports both internal and external URLs.
No, the entity browser does not work for component defaults as the entities are content, not config.
Modifiers allow you to apply styles to an element when a specific class, id or attribute is present.
The Youtube video embed is responsive so it should expand to the width of its parent. If you do need to set a width you can apply the Width CSS property through the Style Builder.