From the aftermath of ww2 up until some time in the 1980s America was the leading manufacturer of... Stuff in the entire world. This was the creator of wealth in America. There were unions that made it so American wages were good for the factory workers, but there were also tariffs and quotas on imported goods so that American companies could sell things at a price to support good wages.

It used to be that a high school graduate could support a family of 5 with a job at a unionized factory, and retire at 65 with a pension that was enough money to take around the world vacations till whenever they croaked. Remember those times? These people had time to go to church and since everyone had enough money there was no need to be upset about anything. At least if you were white, which 90% were. America was great. This was the world of the boomers. This is what Trump's slogan refers to.

By removing the barriers to importing foreign goods, and opening trade with countries that had very bad economies in the 1960s and beyond, companies outsourced their manufacturing to those places- mostly china. But for example after NAFTA gm moved most of their manufacturing to Mexico.

Imported Goods made using cheap foreign labor and environmentally terrible factories undercut American business and lead to massive shrinking of the industrial sector. This is why Detroit in particular is such a shithole now when in 1950 it was the richest city in the world.

Since these other countries don't have environmental or labor protection, plus bad economy, it's much cheaper to make things there, and with no tariffs and "free trade agreements" to lower costs even further, they can be then sold in America and undercut the prices of America made goods. That is how Walmart succeeded at destroying small stores.

This also allowed multinational businesses to destroy the us labor movement by either outsourcing or threatening to outsource any unionized jobs. Since it was impossible to sell stuff for so cheap as it could be made in china and still pay the workers as much.

America went from mostly exporting manufactured goods, to mostly importing. That means a trade deficit where we are hemmoraging money to other countries.

At the same time new immigration laws allowed Third world migration en masse to America, further depressing working conditions, breaking unions, and upending hitherto stable and productive social norms.

As a result of all this, all the stupid white men that used to work in factories have nothing to do except get addicted to meth and opiates. Because wages are lower, white women now have to work also which destroyed the family. There is no more spiritual center due to the multiculturalism necessary to absorb a huge influx of immigrants that have no desire to assimilate to American culture or religion. All of this is why our culture sucks now.

You ask who benefits? The globalist 1% and (((foreign))) infiltrators aka investors. Not the us citizen in general.