> How do you trademark the exact shape of a Lego brick?

You can trademark a shape, actually. It's called Trade Dress[0], and The Lego Group uses it, too. The Minifig, for example, is registered[1].

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

[1] https://trademarks.justia.com/865/37/n-86537461.html