When a government assess a tariff on a particular commodity/import, exactly when is it paid by whom?
By the importer of record, at the time of import. (Short answer). However, sometimes if something is ordered and paid for before a tariff is levied, it may not be subject to the tariff, even though the physical importation is weeks or months later. Not sure exactly how that works or why that is.
Sorry-incomplete: importer of record can be the seller, the buyer, or some broker or agent engaged by either. The net effect of course will nearly always be that the buyer pays the tariff, either directly or as an increase in purchase price to reflect tariffs paid by the seller.
A tariff becomes due at the moment the good crosses the border. It may be ‘paid’ before, at the border, or after but it is due at that point.
This ignores special situations like bonded warehouses which allow imports to be stored without tariffs being paid.
Ultimately, the consumers.
Sorry, to be more specific - a tariff nowadays is imposed for assorted reasons… Generally the complaint is “you foreigners are selling this at too low a price.” The tariff is supposed to equalize the price in the home market. Obviously, the net result is that the cost of foreign goods is raised so the cost of local competing goods can meet or beat that price. So, obviously, the price of any such good to consumers goes up - whether directly, or because that good is used to make further products.
Tariffs may be imposed for “dumping” - the foreign manufacturers are selling below what they charge in their home market because they are overproducing and can’t sell the extra at home; so they will sell for any price, so as to avoid piling up unsold inventory. They may get subsidies from their government, or have other unfair advantages (no health insurance costs for their workers, no union wages, no pollution controls…) They may be instead more efficient producers, and the tariffing(?) country just wants to protect their own producers.
There’s the “national security exemption”, where a country thinks it should not lose the ability to locally make computer chips or jet engines or ammunition… or steel and aluminum, apparently.
Then there’s retaliation - “We disagree with your steel tariffs, so we’ll impose a tariff on soy beans.” If there’s a big enough market outside of the source, then either the farmers need to find different customers for their beans, or if there’s a market glut - they can’t sell them, but the country imposing the tariff pays someone else the same price for soy beans and the consumers don’t suffer. (Depend whether the tariff is on every import, or just from specific countries; retaliation is usually just against specific countries)
It still does not matter. When the goods cross the border, the company crossing the border with the goods has to show the tariff is paid. therefore the cost of the goods goes up, and the consumer ends up paying.
In the case of steel or aluminum, for example, American producers had trouble competing on price. The price of foreign metal goes up, the American companies can charge a more profitable amount for their production, secure in the knowledge they can’t be under-bid. The theory is this gives them the breathing room to modernize so when the tariff comes off they can compete. Often it has the opposite effect - the companies become complacent knowing they have a safety cushion, and eventually may end up unable to even compete at the new higher price.