Funny enough, the mechanic that I use for work on my truck (greater Vancouver area) is the (legitimate) owner of 12 handguns, so it is not just an American phenomenon. I once asked him why so many, as the most he can shoot at one time is one. He said he just likes guns....
I think the Americans are not so much concerned with immigrants who passed through Canada and Mexico legitimately, but more those who the two countries allowed an easy pass through to the U.S. Really, I suspect what Trump et al are doing is trying to divert attention from their own inadequate system. The numbers passing through Canada, then into the U.S. are minuscule in the whole scheme of things. Mexico is quite different of course.
In the 30 years I spent in Canadian policing, I never heard of any hard drugs of any quantity ever going southbound. It was always pot going south, in exchange for money or coke coming back, or in the most recent case I was peripherally involved in, a very large quantity of cash going s/b to purchase a large quantity of fentanyl, heroin and coke to bring back. But that's a few years back now. And I bet, currently, that fentanyl in large quantities is still way easier to get in the U.S. than in Canada. Again, divert attention and blame to someone else.
And to your last point, I don't know how to provide a link, but if you query Dr Andrea Love, Biomedical Scientist Answers Pseudoscience Questions from Twitter (about 5 months ago) on YouTube, she has about a 20-30 minute video during which she debunks (even explains the history on each and how things got so twisted) quite a few topics, vaccines and autism to name a couple. She even takes a dig or two at some of the well known garbage peddlers. Great video.