After over an hour trying to figure out how Capture is supposed to work, the only thing I have discovered is when you capture a page it shows up under Manage Pages but does not show up in the document. Either way, this would be extremely cumbersome if I were building a large document (100 pgs+) and therefore unusable for any PROFESSIONAL purpose.
I have no idea what you mean by PLACEHOLDER text and images so unless it is considerably easier to use than Capture it too would be unusable for building large documents, both in Pages and Numbers.
Again, another unsolvable question so will close close this line out.
Not unsolvable, both Jerrold and I pointed you to where you should look for answers.
The next step would be to further resources, but looks like you have bailed out before you began.
For a start a captured Layout is used to build your document. That is what templates are.
Placeholder text and images are exactly what they say they are. Drag new content onto them and the new content replaces the old. Again exactly what is happening in the templates.