Some other points not specifically addressed:
The regulatory authorities in each country specify their 'take' on transition altitude(s).
Oz uses 10000' (climbing) & FL110 (descending) as the changover point. As QNH reduces below 1013 then flight levels from FL110 progressively become unavailable in order to preserve at least 1000' between aircraft using QNH & a/c using 1013.
USA uses 18,000' as its equivalent to Oz, and then makes FL above that unavailable as QNH reduces below standard. Similar concept but as far as I can see they don't preserve a 1000' buffer to the first availabe FL above 18000' on QNH (or more strictly, '29.92" altimeter' here, since you'll be met with utter confusion here if you start talking about QNHs & hectopascals/millibars).
UK has some entirely random transition altitude that depends on your location & probably the phase of the moon all mixed in with many schools teaching their PPL students to use QFE and the military using using the same. In other words it's not obvious what you should be on where, with whom, and when. Very British in efficiency & ease of use.
Other countries have their own idea on how it should be done. Canada may (or may not) use a system that matches the US but they don't have to.