Primary sources are like witnesses in a trial. Secondary sources are lìke the solicitors. The judge questions the witnesses and listens to the lawyers. Then, he announces the verdict.
Primary sources are data. Historical reasoning has to work exclusively from primary sources. Secondary sources are often how you become aware of and understand that data but they are never data in and of themselves. You could define primary sources reductively as those sources historians will accept as evidence, or more usefully as relevant material deriving from something which is not extant.
As a historian, you are a detective, discovering historical facts forgotten in the historical record for various reasons.
Secondary sources are written by historians who survey/ (detect) the historical record for primary sources that will help distill the "historical relevancy and theory" of the primary sources.
Remembering that the historical record is incomplete and full of holes is helpful. It is the historian's job to try and fill these holes with data derived from methodologies developed by historians. Some work for specific projects, while others are universal.
The standard methodology is a review of secondary sources on a subject and using the available primary sources to support the new project/ theory.
But we must remember that it is tough to reconstruct what life was like 20 years ago, let alone for hundreds if not thousands of years in the past.
I always use the (US) example of trying to explain what life was like to students on September 10th, 2001 and contrasting it with September 12th.