I hate to be anecdotal, but if a healthy, enthusiastic young man goes to his local union hall telling them he wants to be - for example - an electrician, he will be told to go fuck off. Same for plumber, steamfitter, bricklayer, etc.
It doesn't work that way. You have to know somebody or know somebody who knows somebody.
My nephew went through this process, and was after a year or so able to find a job as an apprentice with a NON-UNION construction company. After working his way up for five years or so, the electrician's union finally let him in - as an apprentice. He is doing fine now, getting well paid and doing as much work as he chooses on weekends to supplement his income.
The idea that these trades are not being filled because nobody wants those jobs is bullshit. The "system" is set up to make it difficult or impossible to break into the skilled trades.
Unless you know someone.
In my area (north of Pittsburgh), roofers, landscapers, and other small contractors who employ unskilled workers cannot fill their needs, even at $30/hr. People don't show up, refuse to work hard, or quit after a couple days. At least that's what they tell me.