I've seen it first hand. I've talked with people that have been impacted by it. I've also seen the corporate slight of hand that tries to disguise it. Components manufactured in China, shipped to Mexico for assembly, then shipped to the US to have some decorative panels and stickers slapped on so they can put a "made in USA" sticker on the side.
Go grab virtually anything from around your house (excluding food) and it's more likely than not that that thing was manufactured in whole or in part outside the US. That didn't used to be the case. Coming out of WW2 it was rare for people to own non-domestically produced goods.
This isn't a case of US manufacturing just being so ridiculously efficient that you just don't need many companies making things. That might hold water if most goods were still manufactured in the US, but they aren't. Cars are about the only manufactured consumer good still made in the US, and even they are quite often assembled in Mexico. Pretty much any electronic device is made in China.
Here's a case study, Ikea which is a very popular furniture manufacturer (among other things) mostly assembles their products in Mexico and China. They do have some US manufacturing, but that's largely because they acquired rights to some large swaths of woodland and it makes sense to process and assemble that on site.
To be clear, you mentioned agriculture, and yes, the US does have a thriving agriculture sector, but agriculture isn't manufacturing.
The US screwed up badly when it let basic manufacturing get outsourced. When the factories making screws and transistors are all based in other countries, eventually all the other factories are going to follow them.