Those working in the food production. What do they do with the leftovers or the things that for example don't have the correct shape. they just throw it away or they put it back and reproduce it?

Special training in production is not required, is it?

Manufacturing production work is carried out in factories and warehouses all across the country. Workers may provide general labour by performing various tasks which support and help other, more skilled employees. Others may be skilled technicians who put in place, test, operate and remove manufacturing systems, machinery and tools.
General labourers are often required to load and unload products and materials, clean equipment, feed material into machines and observe equipment to spot malfunctions. They are normally supervised by more highly skilled technicians. Machinists, lathe operators and machine operators maintain, use and monitor manufacturing technologies to create products. They will have specific training, certification or experience
Nearly all manufacturing production work requires a certain level of skill. If you are working as a manufacturing production technician or operating equipment, it is usually necessary to have measurement and maths skills, as well as an understanding of manufacturing technologies
Most general labourers receive on-site training and learn how to carry out duties from more experienced workers. Training can also take place in the form of an apprenticeship. The average salary of an individual working in production in the UK is £21,334.