Simon uses his spiral power to write a story, and this story becomes its own 2 dimensional universe.
In essence, while crafting his story and the world within it, Otoko Simon utilized his Spiral Energy to connect his own universe with a 2D universe. Alongside the power of his voice, this connection enabled that universe's own Simon, "Garlock," to perform "spatial teleportation," allowing him to cross into Otoko Simon's dimension. When Otoko Simon tore the manuscript, it created a "gateway" linking the two dimensions. Additionally, any modifications made to the manuscript directly influenced the reality and storyline of the 2D universe, notably steering Garlock towards meeting Kamina to help him grow as a person.
In simpler terms, the primary distinction between a 2D universe and a 3D universe is essentially between fiction and reality. By writing with a pen charged with his Spiral Energy, Otoko Simon crafted a fictional 2D universe that he could manipulate at will through his script within his 3D reality. This implies that traversing from a lower-dimensional universe to a higher one in TTGL involves moving from a fictional realm to a real one. Each dimension above perceives those below as fictional, with each successive dimension layering further into a greater reality.