Explain Like I'm Five

So whose smart idea was it to think tariffs would not hurt the american people.>

They are easily identifiable by red baseball caps.

The intended goal of import tarrifs is to aid local manufacturing. It's one of the major tools of mercantilism (1)

It's a tool that can work. The EU, for examples has had import tarrifs for decades now. Very similar to the ones the US is implementing now: rate depends on country of origin, as well as the type of product.

But it takes time, as reshoring industry means building infrastructur, redoing logistics, educating people. And industry has to trust that the policy won't be reversed in a couple years time, before wanting to invest in the US.

So whose smart idea was it to think tariffs would not hurt the american people

It's essentially the reverse of liberalism, something that was/is popular to hate accross the political spectrum. It's what people were asking for. So as a direct answer to your question: popular demand.

Seller does not have to raise the prices.
Importer (US company) pays the additional import tax (tariff) and then covers that by rising consumer prices.

The exporting country doesn't charge more, the increase comes from the duty charged as it lands in the target country. So unless the exporting country drops its price to compensate (which often can't happen at the margins commodities use) then it raises the price locally to pay for the tariffs. If there is a local source, that source can sell anywhere between their usual price and the new price level of the foreign product. Usually what those companies do is just raise their prices to almost the level of the tariffed goods, causing inflation, which again the consumer pays.

The theory is that it gives an incentive to invest in production facilities locally, but what this wingnut has done is change the policies an uncountable number of times and scared the hell out of anyone putting money into factory expansion, because they have no idea what grenade he's going to toss into the foxhole next. Cap that off with firing the trusted sources of information in the administration so nobody knows what's true and what's propaganda, and you have a recipe for stagflation.

It's going to be grim unless he has a heart attack or otherwise mysteriously croaks, because it doesn't look like Americans are smart enough to run him out on a rail, despite his obvious pedo issues.

Good luck!