Ask HN: When do tariffs get levied? Who specifically charges them and how?

I'm trying to learn more about the operational side of tariffs.

- How do they get calculated? - Who calculates them and what tools do they use to calculate them? - When is the calculation done, and who pays for them?

What I'm imagining:

1. The Exporter want to ship something to America from Vietnam.

2. The shipping company loads it up and sends it to LA.

3. Item arrives at the Port of LA.

4. The Importer collects the item (or bulk items), and the port's tariff collectors inspect the shipment, calculate the tariff based on their valuation and tariff chart and charge the importer for the goods before they can be released.

Is that right? How does the importer determine the cost ahead of time - is there an online calculator? How does the import department ensure that the value of goods isn't being low balled?

Thanks!