To paint the American landscape a bit more, the majority of what are called "union avoidance" activities are legal: since employers are employing the workers, they have a captive audience, and use that to the full extent of their ability, usually with the help of outside legal consultants. A lot of what goes on is surveys, videos, forced informational sessions, etc. Much of it can be compared to targeted public relations. Even when an employer stays within the full bounds of the law, they're usually substantially advantaged over the labor organizer: full information about the workforce and a captive audience goes really far.

So where do the violations of law come in? A lot of it comes from very murky, subjective boundaries: employers can't threaten to shut down a plant or time wage increases in a way to interfere with a union organizing campaign. But a lot of what union avoidance specialists' work involves is consulting at exactly how to do the above in a legal way. It will end up in court, and if the consultants did their job right, the employer will win.

Beyond that, you get into activities like firing workers purely because they're involved in union activities: here the labor avoidance consultants will sometimes help with building a paper trail to target workers viewed as problematic. Another illegal activity is spying, either on union meetings or workers and organizers when they're off premises. Oftentimes, employers want to do this on their own volition, and union busting consultants actively work to curtail this (or at least make sure any firings happen in a legal, documented way).

Generally you need to have a minimal number of employees that form the committee of a new union. You register at the Court, inform your employer. Anyone who wants to be represented by the union can join you. The employer must inform you about organisational changes / salaries / etc. You can add your union to a confederacy of unions, biggest of which has a place at the tripartite table during Labor law updates - so it's the state, employer groups and labor unions deciding about yearly wage increases.