They just look for cheats.
If you aren't cheating you don't need to worry.
They have algorithms largely on expenditure.
If I buy a property or a holiday home and have been declaring 20k a year and claiming benefits they will want to know why.
They will look at the year you bought the property and ask where the income came from.
They will look at your bank accounts.
They aren't worried if you have a receipt for a van MOT twenty years ago. They will just say 'you declared an average of 20k over ten years - how come you have bought XYZ big ticket items in year 7, 12 and 20?'