The unions negotiate for "All workers" under some kind of umbrella terms that has only sometimes specific prescriptions towards individual companies but always towards industries as a whole. They can dictate things like medical, vision, dental, sometimes day care. But they can also dictate things like work hours, safety, etc.
That all sounds gravy, but what if a company like Tesla wants to dictate 60 hour work weeks for all it's engineers. But that goes against Union Law, so they can't. But what if that's just how the people in the company, and people within that company want to work. That's how Tesla works. When you are getting of job at Tesla, you know exactly that you are signing up for long work hours. It's a company with everyone putting in 70 hour weeks for around 2 years, and springboard into another startup at a higher role, or more cushy FANG.
To give another example from another industry that would not work is the oil fields. We have the OCAW union here in the US but get like 1/4 of what the non-union jobs get. Because OCAW dictates work hours, and a salary but not time to rig. the non-unionized guys go out and work for 18 hours straight, sleep on a the rig for a bit, and get back to work. Again, not allowed under union contract, but if you do that for a couple days and make a bunch of money. Because we have worker pay, OSHA, etc. There is no need for unions here because the minimum labor standards for overtime pay and safety are set by federal standards not the "Union".
"Collective Bargaining" has lost it's sense of "Collective" at the scale it has grown. True collective bargaining has always been by the individuals themselves. Forcing Tesla on a blanket union contract instead specific clauses and prescriptions that they need to implement to be compliant is legalease non-sense.