Protectionism only works in specific situations. For example you have a competent workforce and instead of making them work in a foreign country because that's where the multinational companies are, you introduce protectionist policies to keep them in your country. Getting to this point requires decades of focused efforts. You don't just introduce protectionism and get results automatically. Protectionism doesn't work because you have to make it work.

It's a bait and switch. Get foreign investors to build companies in your country. Let them pay for training. Children can now go to school and university. They become adults and gain experience during their job. They now want to leave the country for higher incomes (jackpot). Then finally give them a reason to stay. The Chinese government has amassed an insane amount of money and growth during their business friendly periods and is now able to deploy it for protectionist measures.

The reason why protectionism is a bad idea is that it's commonly used in incompetent governments that are prone to corruption before the country is ready. They instate the protectionism before the country has formed a competitive industry. The end result is that consumers have to pay hefty tariffs on foreign products that are almost impossible to replicate domestically like GPUs or CPUs. Alternatively consumers are stuck with inferior domestic products because there is no reason for the domestic monopoly to improve without competition.