When you submit the application for probate you need to list out the assets and the liabilities. If the deceased was well organized, they will have compiled a list of assets which also states where relevant documents can be found. Often these will be stored in a safe or perhaps with the bank, solicitor or accountant.
Unfortunately, in many cases, it is not always that straight forward and listing all the necessary information will require persistence. It’s common for older people to have papers in the attic, or perhaps at the bottom of a cupboard for example. Younger people, assuming they plenty of years to live, may not have any records at all.