The general answer to this is that the more industrialized a country is, the more of its people will work in the tertiary (or higher) sectors and fewer will work in the primary and secondary sectors.
Before going on, let us define our terms. An NIC is a newly industrialized country. An LEDC is a less economically developed country. An MEDC is a more economically developed country. The primary sector of the economy is the sector that is concerned with extracting resources from the earth. This includes farming most of all but also things like mining and logging. The secondary sector is the manufacturing sector. The tertiary sector is the service sector, in which people do not make goods but rather create services that can range from hair cutting to the creation of movies and TV shows.
In an LEDC, industrialization has not really started. There is a very large primary sector. Most of the country’s exports, if any, come from this sector. There is little manufacturing, meaning that the secondary sector is still small. There is a fairly large tertiary sector that typically provides low-cost services, often on the “informal economy.”
In an NIC, the secondary sector has just started to grow. This means that the primary sector is large but is shrinking. The secondary sector is small but growing. There is a smaller tertiary sector in such an economy. Some of the people who were in the informal economy in an LEDC now have formal jobs in the secondary sector. A smaller, but formal, tertiary sector is starting to arise to provide services to those in the secondary sector.
Finally, in an MEDC, the tertiary sector dominates. This is why such countries are often called “post-industrial.” The primary sector may produce great value, but it is highly mechanized and employs few people. The same goes for the secondary sector. Most people work in the tertiary sector in such an economy.