Why tariffs (taxes on imports) are bad, in a nutshell:
Costs are passed along to consumers. Maybe the foreign company can eat a portion of the costs in reduced profits, but profit margins pale in comparison to tax rates.
Tariffs apply to consumer goods as an obvious cost, but also apply to raw materials as a hidden cost, increasing the cost of production in the US for the very firms it's supposed to help.
Retaliatory tariffs from other countries in a trade war stifle opportunities for American businesses and thus harm American workers.
Less competition means more room for corporate fat cats to raise prices, generating new profits and lining their pockets with zero guarantee the workers who pay the new higher prices will see more pay.
When the country was funded by tariffs, they were generally used only for revenue, and not as a trade war weapon. The government was also a lot smaller, and yet even then, the tariffs were a drag on the American economy despite the relative boom in prosperity between the Civil War and WW1.
A podcast video on the topic of Trump's proposals in particular:
thanks. this will no doubt be better than the other things I am looking at from compromised news sources.