No source, but in general it worls like this: each indiszry has a leading union and a coreespondong, leading epmployer association. Those two collectively bargain for all represented employees at represented employers, e.g. salaries, workinng hours and so forth, up to a certain salary range. In one case, the metal and electric industry in Bavaria, those salaries can go up to close to 120k Euro for a 40 hour contract in the highest wage bracket (those salary table are public).
In addition to those general agreements, worker councils and thwir individual employers can, and routinely do, negotiate supplental agreements, e.g. extended working hours, christmas and vacation bonuses, overtime (up to the possibility to save up for early retirememt or fully paid sabaticals).
And since Covid, those agreements include up to 40% of remote work where possible. Before Covid, working councils, and thus unions as the majority of council members are union members normally, where oppossed to that. Which kind of makes sense, since most union members tend to be blue collar folks with serious problems when it comes to taking work home. And those priviledged, like in no shift work, bettwr pay and less physical labour, office folks (I am one of those) already had enough benefits (which I get). Covid changed that, even if I have no idea how that came about, I wasn't working in collective bargaining environment during that time.
Overall so, unions turned out to be way more flexible and pragmatic, as were employers, that some people, certainly myself, expected.