this is a complex issue, to say the least. the economies of each side are deeply intertwined and the back and forth trade has greatly benefited both sides, even if assymetrically. it is beyond simply sending products back and forth.
for example, china holds $1.11 trillion in US Treasury securities. if one side's economy drops, both sides drop. that is the nature of global economics now. here is an article that shows some of this.https://www.aljazeera.com/ajimpact/markets-sink-trump-presses-companies-leave-china-190823165734640.html
but this more important than just trade with china.
US economic wealth depends, indeed assumes, a global market place with set rules and expectations for participation from leading countries. trump, in his childish and simplistic approach, is dissolving and eroding the very system that has placed the US in the world lead. this involves more than a trade war with China. through trump's heavy handed approaches to economics with europe and see that the world system of economics is being compromised by his decisions. the talks in europe among the G-7 are crucial right now since at stake is not trade, but the global system that underpins the rules and behavior for global trade.
next, on the topic of technological flows. this too is complex. it is not correct at all to say all of China's growth in technology is from stealing. indeed, to state so is to be show profound ignorance of a complex process.
globally, technology by leading countries has intentionally been handed off/pirated/copied for centuries by less technologically skilled nations. this is a good thing overall.
as originally articulated by the japanese economist Akamatsu in the 1920s, in terms of technology that drives economic sectors, the world NEEDS a variety of countries to be located differently. for example, if EVERYONE produces cars, where does the food come from? textiles? the world needs a variety of countries that produce a variety of goods.
simply stated, the product cycle (i used to devote weeks of a grad courses to teaching this crap) is agriculture=>light industry=>heavy industry=>consumer durables=>high tech. different nations align on different sectors of this process.
as countries move through the product cycle, they cast off older technology and followers snap it up. as the US stopped needing advanced tech in textiles, it sold/cast off that technology and focused more on heavy industry (steel) which is more profitable. this allows a division of labor to take place, which is good for the global economy over all. sometimes this technology is given, other times copied, other times stolen.
oh, and it is quite common for the new-adapter of an old technology to improve on it and take it further than it was originally designed to be. innovation.
further, unless lesser developed nations (called "late starters" in this theory) get benefits such as cast off tech, they have no incentive to joint the world system. if they dont join, the system fails. if the system fails, more advanced countries' economics struggle.
even the US went through this tech transfer less than two centuries ago. the US industrial revolution of the 1800's was made possible by US technology piracy at the time. the US stole the secrets of textiles production from the Brits. this paved the way for light industrialization, which lead to heavy industrialization over time.
something else to keep in mind is that from 1946 on, japan has frequently copied and stolen technology from the US.* but since the US likes Japan as an ally, it allowed such behavior to take place. China is not an ally, so now the US objects.
it is NOT that china is "stealing" and unfair that has suddenly outraged US leaders. it is that the US does not view china as an ally politically. as such, the US is less tolerant of the behavior.
*(by the way, the practice of "dumping" was pioneered by the US long before Japan and China did it. Indeed, US business leaders in the 1940s after WWII, showed and told Japanese corporations how to dump products. they did so since the US wanted the japanese economy to grow VERY quickly and help the US in its Cold War against communism.)