grammar - Why can "which" and "who" be dropped in some sentences?

These two sentences are trying to express the same thought:

1. Now we examine the trunk, which is also called the boot, especially outside North America.




2. Now we examine the trunk, also called the boot, especially outside North America.

First, are they grammatically correct? If so, why is it ok to drop the "which is" from the first sentence?

Similarly with "who", we can say "This is Robert, who is also called Bob." or "This is Robert, also called Bob."