Answer: Trump is obsessed with this idea of a trade deficit between the US and Canada/Mexico/China. He mistakenly believes that because the US is a net-importer this makes the US a loser in a trade war, and that the US is therefore subsidizing those economies. (Total economic non-sense)
Exactly what Trump's plan is isn't clear because despite his rhetoric on the campaign trail he refuses to elaborate on his long-term plan when asked.
Given his history of wanting steep tax cuts it's possible that this is an attempt to generate revenue to deal with the deficit and cut income taxes.
Trump is a big fan of President William McKinley who was known for his tariffs. Trump even went so far as to rename a mountain in Alaska back to Mount McKinley in his first week in office. So that gives us some insight to his thinking.
We can assume then his plan might be:
Raise tariffs on foreign trade, therefore forcing the exporting countries to pay 25% or more taxes on exports for the privilege of exporting goods to the US. (there's a significant flaw here, I'll get to it in a moment)
Use this income to lower the deficit and justify cutting income taxes that hurt rich people more. Continue to hack+slash at existing government spending.
Encourage US businesses to hire US workers and bring manufacturing back to US shores.
That sounds all fun and good, so what's the problem?
Trumps plan seems to be centered on levying tariffs which will force foreign countries to pay extra taxes to the US to export their goods for US consumption... except that's not how tariffs work.
Tariffs aren't paid by the exporting country, they are paid by the US importer, so the actual person paying the tariffs will be the US consumer in the form of higher prices.
Donald Trump either doesn't understand how tariffs work, or is willingly lying or ignorant of this fact.
If Donald Trump follows through on his plan and continues to double down on tariffs to get his 'revenge' on these countries for Fentanyl, immigration, or whatever excuse it is today it will cripple the US economy.
Prices for everyday items will keep going up, because it's the US consumer that ultimately pays the tariffs. US domestic goods will also go up in price, as the local producers will know they can charge more and get away with it (because that's how capitalism works).
This will drive inflation a lot harder than now, and the average person in the US will suffer. Trump will no doubt blame China, Canada, Mexico, or Biden/Obama for these problems instead of taking responsibility for his own mistake.
Trump seems to have this idea that we can return to a pre-1920's model of economics where Tariffs will pay for the US government. Except the Pre-1920s the US federal government was far smaller than it is now. There was no interstate highways, no airports, the average person had a 6th grade education. The US military had no airplanes, no tanks, no trucks, and no missiles because those things hadn't even been invented yet. No social security, no medicaid.
It would be impossible to fund the US federal government with tariffs alone, the math doesn't work.
The current tariffs will only account for 5% of the US Federal budget, but the tariffs will need to be 100% or even 200% on imports to make a dent in income tax revenue. This is turn will cripple in the import market and lower the tariff income because no one will be buying imports anymore.
This whole plan is insanity
One economist described the situation as: "A man that lacks a basic understanding of global economics and mechanisms is now in charge of the US economy wielding tariffs like a bull with a bazooka in a china shop."
Trump seems to think that levying tariffs will cause the foreign countries to bend the knee and give whatever concessions he wants rather than pay taxes on exports. He even bragged about making Canada the 51st state and giving Canadians "a great tax cut" in the process. He's treated this like a leveraged buyout, crippling another business financially so he can sweep in and buy them out looking like a savior.
Except for the major flaw that he doesn't understand how tariffs work...
Canada + Mexico seem to be banking on the idea that if you tariff the Red States and by extension hurt Trump supporters directly, one or more of them might finally stand up to Trump and get him to stop.
100% tariffs on Tesla for example might get Musk to personally step in and confront Trump on his tariffs.