Is all the packages are custom objects objects are packages are created to work with standard object componenets as well ?
A package can include its own custom objects and also custom fields added to standard objects. Other components including Visualforce pages and triggers (so Apex code) in the package can then reference both the package's own custom objects and custom fields and the standard objects and fields (and other standard components).
A design decision for a (managed) package is which Salesforce license the package will work with; if the package only references e.g. Account and Contact then it can work under a Salesforce Platform license users but if it references e.g. Lead and Opportunity then it will require Salesforce license users. (See What is the difference between the "Salesforce" and "Salesforce Platform" licenses? for more detail.)