Mexico and Canada don't have to pay anything. People living in the US have to pay the extra 25%. Which means lots of stuff will be more expensive so people probably won't buy as much, which will mean Mexico and Canada can't sell as much. So Trump puts pressure on other countries by hurting US citizens.
Can you explain what you mean by MX and CA having to pay 25% more? A tariff is an import tax paid by the company doing the importing. So American companies that were importing those goods will now have to pay a 25% fee on top of what it previously cost to import them, which means that they will in turn raise the prices of their products to account for this new loss that they unexpectedly started incurring. The end result of that is that by raising their prices, us normal American citizens will have to pay more for those goods.
Foreign countries don't pay tariff costs. We have no way to enforce that, nor does it make sense for them to do so.
