Canada is one of our largest trading partners... in the context of "who we do business with", they are in the top 2. So saying "we do very little business with Canada" is simply wrong, lol. It makes no sense at all.
Country A sources 99% of its economic activity internally, and 1% goes to Country B. Country A trades with no other country. Does Country A "do a lot of business" with Country B? That's relative. Compared to their total trade? Obviously, it's 100%, they trade with no one else. As a percentage of economic activity? Clearly not, it's a relatively insignificant contribution to their economy.
Most US trade is internal to the US. Canada is simultaneously one of the US's largest trading partners and a very minor part of the overall American economy. That's Trump's point, he actually goes on to state a figure ("maybe 4%") which overstates Canada's importance to the US. Conversely, the Canadian economy is far more dependant on the US than the other way around, which Trump also went on to point out.
The trade relationship is of asymmetric importance to both countries.
, Trump understands and sees it as leverage. Feel free to have a conversation on whether or not you think that's good policy.