You're right that those countries don't have a government mandated minimum wage. That doesn't mean there is no minimum wage in practice though.
Scandinavian countries have massive powerful trade unions that work to negotiate standards and wages for everyone. Even if you're not actively part of a union, your employer will almost certainly use Collective Labor Agreement negotiated by the trade union. This CLA guarantees certain things like vacation time, minimum wages, break times, etc.
Now you could not use a CLA drafted by a union and instead try to fuck people over with your own contracts. That's not very likely to work though since these countries tend to have strong social welfare systems coupled with free healthcare. So if people quite due to shitty pay and/or hours, they can still make do with social benefits. Also people here in Finland at least, know that for most jobs you absolutely want a CLA drafted by one of the unions because that CLA protects you.
Most companies use the CLA's as a base and allow employees to negotiate better contracts if they want to. E.g you can ask for less hours and more pay, the CLA generally only guarantees minimums. Now an employer can always try to renegotiate the agreement with the union for their workers, but you better be damn sure you treat your workers right since if the unions decide that you're not, they will fuck you up.
These unions are massive and hold a ton of power. In Finland over 70% belong to a union and it is not uncommon for them to strike if the employers are trying to fuck them up. The best example of this is the recent chaos with the Finnish Postal Service.
To give you an idea on what happened, the postal service tried to renegotiate the CLA they used, which would have resulted in worse pay and hours for the workers. Union was taking none of it and went to strike. The postal service tried to get around this using questionable means which broke International Labour Organizations rules. This led to a ton of sympathy strikes which eventually ground the nation to a standstill.
Public transportation froze, flights were cancelled, trains didn't run, all goods transported by postal cars were boycotted by unions this led to stores not being stocked up as they should have for example, ferries and cruise ships under Finnish flag froze. Among numerous other things. The unions eventually won after inconveniencing thousands of people and costing corporations tens of millions of euros.
The workers didn't get shafted and their rights were protected even without a government mandated minimum wage.