I don't know much about how most unions work, but at least TV & movie actors and writers, classical musicians, and professional athletes have unions that work as task_queue describes. It's hard for me to imagine a programmer's union that didn't work this way getting off the ground.

Here's an example: The Screen Actor's Guild.

Matt Damon is a SAG member but he does not get paid scale. Scale is a minimum wage set by the union but if you're a big enough star, you negotiate your own wages.

Depends on the union. Some allow for negotiation, some have a scale which you can work with.

Bonuses and supplementary pay are usually not determined by unions and are up to you to negotiate in this situation.

Evidently this is highly industry-dependent. My experience is in the US auto industry, where all union workers' pay and benefits are fixed by what the union negotiates with the company. (At least, they were when I worked in that industry.)

Pretty much all the unions in industries where there are recognized hyper-elite workers work this way (SAG s mentioned in a sibling comment, most of the major professional sports players unions, etc.)

Most other unions don't work that way.