I assume you've read
Sources and Citations in Version 7 (for Upgraders)?
A Prepared Citation is a combination of a source definition and associated citation-level data created ready for use in
Source-Driven data entry.
A 'Prepared Source' (not sure where you found that terminology) is (I assume) just a Source that has been created, by e.g. adding a source via the Source Records Window or via the Create Source/Prepare Citation dialog without entering Citation data.
I have selected a prepared source and tried to add a prepared citation to it (Add > Source/Citation > Prepare citation to existing source...) without success. I can enter the information, but it does not save.
Where have you looked? The most recent Prepared Citation (there is only one in existence at a time) is stored in the Header Record.
If you don't use ASC or DEAs, it sounds as if you may be using Add Citation within the Property Box, and not using Prepared Citations at all. To take advantage of them, you have to use either ASC, or DEAs, or Copy Citation (all at the bottom of the Prepared Citation Window). If you use Copy Citation, you can then use Paste Citation within the Property Box Source Pane.
Note: whether you create a source per page, or put page data in the citation 'where within source' is up to you, and will probably differ depending on the source. If a source was a published Book,
I would put the page number in Where within Source (i.e. at the Citation level); whereas if a source was a Census I'd create a Source per Census Entry.