if you make millions of Legos in mass production it reduces costs to make them dramatically to a point where the Lego Group has basically no operating costs to make them anymore
That's how economies of scale work in general, across many, many industries.
On a somewhat related note, your Lego example is more gloriously intricate than you may realize. So, you're spend a lot of money to make a machine to produce Legos at close to zero cost. What happens if someone the next city over thinks they can make a better machine and undercut you?
One way to protect yourself is with the law. You set up intellectual property protection for your Legos and sue everyone who makes "Lehos". This works for a while.
But problems come up. Intellectual property protections have a time limit. They also have a jurisdiction limit, as some guys in a different country, say, Xhina, don't respect your country's laws and start making those Lehos.
What do you do? How does your company survive?
Well, you can leverage the other valuable part of your company, the brand reputation, to do things that Lehos can't, like make deals with other intellectual property holders to make themed Lego sets. So, you strike deals with Disney to make Star Wars and MCU sets, with Warner Brothers for those Harry Potter designs, with Microsoft/Mojang for Minecraft Legos (because they're a perfect fit). That's something that some random plastic injection mold company in China can't do. You're motherfucking Legos, not dipshit Lehos. You can do that, as well as open company stores and theme parks that are tourist destinations.
So, Legos survives, and not just that, but prospers.