How has the US been changed due to globalization? What caused or contributed to that change?
Globalization has wrought many changes in the United States, as your question calls for, some good and some bad. Let's look at some of the changes and the pros and cons.
The United States has flipped from being a mostly manufacturing economy to being a mostly service economy. I have read various statistics on this, most recently, that we are 70% a service economy and 30% a manufacturing economy, with the converse being true thirty or forty years ago.
We have more and more goods available to us from other countries for a number of reasons. Those goods can be more cheaply produced in most of those countries because, among other reasons, labor is cheaper and there are fewer safety regulations to protect workers or consumers. Thus, the American consumer is purchasing more and more foreign-made products and the factories that made many of those products in the United States have become still. This caused us to shift our focus to service industries, providing in ways that cannot be farmed out to other countries, for example, medicine and healthcare. Food and drink are primarily service industries as well, to the degree that they are provided in restaurants and bars. Housing is a kind of service industry, too, with plumbers and carpenters and electricians providing their respective services. Our willingness to enter into trade agreements has eliminated the tariffs that made foreign-made goods more costly, a form of protectionism that some Americans would like to see return. No matter what one's position is on this issue, the fact is that the elimination of tariffs made the American market wide open to foreign goods. The advent of the Internet has also contributed strongly to this change, since goods from all over the world are available to consumers with just a click.
All of this has changed the employment landscape dramatically. People used to have decent-paying jobs in steel mills or making clothing, furniture, toasters and televisions. Those days are gone because we buy steel from overseas mostly, and the foreign manufacturing has taken those jobs from us. What is left are largely service industry jobs that do not pay particularly well, for example, working at a fast food establishment or a chain restaurant or providing cleaning services. The service industry jobs that pay better are not particularly high-paying jobs, in healthcare and education, for instance, and they require more education than many people can afford today. Teachers and doctors, while not starving, are seldom much more than middle-class today. High-tech jobs require training and education, too, and those people who have lost their manufacturing jobs over the years have had a hard time making a change because of age, a lack of training and education, and a lack of help from the government that wrought these changes.
Three advantages that I can point to are the availability of affordable goods to consumers, the opportunity the United States has to sell those goods it does excel in making all over the world, and the prospect, perhaps, of greater domestic security to the degree that having trading partners everywhere encourages diplomacy instead of saber rattling. Neither China nor the United States could possibly afford to wage war on the other, since our economies are inextricably intertwined now in trade.
Two disadvantages are the present lack of upward mobility for Americans in the lower socioeconomic classes, since their means of rising through good-paying manufacturing jobs have gone by the wayside and an increasing dependence on foreign goods, which does make us vulnerable to the vagaries of other nations. For instance, we have not until quite recently been anywhere close to energy independence, which has made for national concern when various events have occurred in the nations that have traditionally provided us with gas and oil. While there are significant advantages to trade, there is something to be said for self-sufficiency.