Taiwanese is another official version of the chinese scripts. Because of the nature of it, chinese script act more like a language than something like alphabetic writing system, with different language using a different "dialect" of the script. And just like language, new characters pops up all the time or slowly morphed into a new form as needed by the language using it. The countries that use chinese script then decided that they need to standardize the "language" so that people can communicate better. As we can see, the two china disagreed on the approach and do their standardization independently to each other, with mainland china simplifying the old "word" whenever they could and the taiwanese preserving more of it.
Japanese kanji being similar to taiwanese is because both country never sought to truly overhaul their characters unlike in mainland china, so their script resemble more of the old chinese script, but the over a thousand year split between the two still mean that they've diverged considerably from each other.