People have fixated on the word "union", but when a group of white collar people get together to set standards and gatekeep them it's called a profession. In the UK, this is usually structured as "chartered". It covers other types of engineers (see the repeated discussion of whether software engineering is engineering; iron rings in Canada; etc) as well as accountants and surveyors.
Older craft professions had guilds. Lawyers go back still further in history and have their "bar" exams.
But in all cases it requires imposing a costly barrier to entry in exchange for not having to prove yourself over and over again to potential employers. Nobody makes a lawyer do fizzbuzz before hiring them.
...because the state Bar Association has required them to pass a test just to show up. In most states the same organization requires a degree from an accredited law school just to sit the test (though I think there are like 2 states that let anyone sit the Bar Exam).
Put another way, everyone has to do the fizzbuzz leetcode test just to be in the industry. But you only have to do it once.
Older craft professions had guilds. Lawyers go back still further in history and have their "bar" exams.
But in all cases it requires imposing a costly barrier to entry in exchange for not having to prove yourself over and over again to potential employers. Nobody makes a lawyer do fizzbuzz before hiring them.