Lego protection was based on patents. They tried to play the "make a slight variation and extend the patent" game but were slapped down. Now anyone can make Lego-compatible blocks legally, though of course they can't identify it as Lego. (Although based on the various ones I've gotten as presents, Lego never had anything to fear. The knockoffs sucked before the patent expired and they didn't get any better afterwards. It's still a terrible idea to let knockoffs mix with your real sets, and that's 0% Lego "purism" and 100% pragmatics. Maybe there's a good specific knockoff somewhere, but the odds seem poor.)

TTC protection is based around other IP constructs. Here's a good sample link from a source that seems to know what is up, which I link to for the legal analysis rather than the details of a 2009 court case: https://www.gamedeveloper.com/game-platforms/exclusive-i-tet...

Your information is outdated. Cada is at parity with lego now, they have even introduced brick designs that lego later copied (flip flop technic beams). The rest of the Chinese brands are similar, and even aliexpress mystery bricks are serviceable. Sometimes their color matches are not exact, but lego had the reddish brown issue so that's also at parity.