Are basements a thing?
As others have said, yes, basements are common, at least in all the houses I lived in (Northeast US). But we called it the cellar, and everything below grade was called "down cellar." I still use the term, even after living in the UK for 40 years, but it doesn't come up much!
Our cellars were spacious, useful for storage, and, once, even a playroom, but never "finished" (converted into proper living space). It was always warmer in winter and cooler in the summer.
I've never lived in Florida or the Deep South, but I know that some areas are only a few feet above the water table, so a lot are built on slab foundations , no cellar.
Do you get annoying older kids on Trick or Treat night?
Used to when I was a child in the 60s. Mainly teenagers.
We were forbidden to go anywhere near Halloween because I grew up in a strict Christian sect, so we also were bullied at school around Halloween because we couldn't participate in the school-mandated Halloween decoration sessions. And the neighbors thought we were strange that we didn't participate in trick or treat.
From what I know from relatives in the US, trick or treating door-to-door is not common anymore (no one knows their neighbors well enough), so parents take their little ones to arranged Halloween parties. (Just what I have heard about the areas where my relatives live).
What one convenience that is common in the US do you rate above all others?
As others have mentioned, screens in windows (proper sturdy metal screens) are essential. Another "convenience" is AC. Bad for the planet, but essential in the summer in most of the US (even parts of the Pacific Northwest now, which is crazy!)
And dyers. Some places forbid hanging clothes out (mainly HOA areas, Home Owners Associations), so a dryer is essential.
Does no one ever steal from those outdoor post boxes?
I've heard of it happening, but stealing from mailboxes is a federal offence, so it's not a risk most people would take.
And do newspaper delivery kids really throw papers onto the lawn or porch as seen in films?
Used to, all the time, and we often made money this way as kids. By the 80s and early 90s, though, most of the routes around where my family lived were taken over by "man with van." But, they would still throw the newspapers! These days, I would imagine most people don't even get a newspaper.
How can you tell that someone is American or Canadian?
I find it easy to distinguish the accent, but mainly in older people now. (I think YouTube and online media in general has "flattened" out a lot of the accents). But, I had relatives in Maine, and there were always Canadians around there, and the Canadian accents were completely different from the Maine ones.
Probably the easiest way to tell is the pronunciation of "out" - very different in "Canadian" (although the Canadian accent changes as well, according to where you are, as it's such a vast place). Newfoundlanders sound nothing like Acadians or Quebecois! And none of them sound like people from Maine!