i dont practice immigration but it is my understanding that it takes much longer to obtain a green card in the USA if you do not have a formal education that either makes you well qualified for a specific position at a US company (H1 visa) or alien of extraordinary ability (O visas; usually Phds and beyond).
categories fluctuate in the USA but right now nurses is in great demand so it is easier /faster to obtain visas if you are a nurse wanting employment in the USA...but again, that is a trained profession.
...I kind of leave out the US out of immigration conversations because it is basically impossible to immigrate there unless you win the Green Card lottery. lol.
I will tell you what I know from the research that led me to that conclusion. I many be misremembering some of it, since I read up on this a few years ago now.
You either need to have a job lined up for an H1 (in which case a college education is not mandatory iirc, only the job offer, proof that you are qualified for the job, and a positive labor market opinion, meaning there is no readily-available American who could do the job in question) or you need to have won some kind of accolade/international acclaim for an O visa (college education not mandatory -- it's the awards/opinions of press or peers in field that matter). The former is nearly impossible unless you happen to be employed by a company that runs a transfer program and has the legal clout to make it happen. And the latter is unlikely for most.. you have to be truly exceptional to get that visa.
ETA: I just looked it up and H1-B visas have fuzzy requirements but apparently it is assumed that a bachelor's degree is the minimum for a "specialty occupation". However fashion models are frequently given this visa on the strength of their looks (!)
In order to get a Lottery visa (i.e., be eligible to enter the lottery), you only need a high school diploma or the equivalent in your country, iirc.
what does Australia mean by "trade"?
A "trade" in UK English (i.e. in Aus English too) is someone like a plumber, mechanic, carpenter, boilermaker, gasfitter, turner and fitter. That sort of thing. Someone who has undertaken an apprenticeship under a tradesman, has worked under their supervision for X number of years, and has been granted tradesman status (i.e. their skills have been formally tested and recognised) by the relevant guild/union.
I agree that medical designations will always be in demand. You cannot go wrong with a nursing qualification. The nurses that are in the most demand in Canada are the licenced practical nurses... who do not need a degree to be licenced. You can qualify within a year at a community college for that career.