There are different models of union. IIUC, Nordic countries use sectoral bargaining: [0]

Sectoral bargaining is when a union covers an entire sector of the economy, e.g. telecom, electrical, aviation.

In sectoral bargaining, companies don't fight unions as hard. ("We'll have to pay our workers more, but all our competitors will too, so we won't be at a relative disadvantage.")

[0] https://en.wikipedia.org/wiki/Sectoral_collective_bargaining