How did manga as we know it today come to be? While researching my own question How important was the invention of blue towards anime?, I landed relatively quickly on Hokusai. The page states that Hokusai is incorrectly considered the precedent to modern manga.


  The largest of Hokusai's works is the 15-volume collection Hokusai Manga (北斎漫画), a book crammed with nearly 4,000 sketches that was published in 1814. These sketches are often incorrectly considered the precedent to modern manga, as Hokusai's Manga is a collection of sketches (of animals, people, objects, etc.), different from the story-based comic-book style of modern manga.


And considering that the first 'manga' was more art then manga as we know it these days What is the first ever produced anime and manga?

How did manga as we know it these days come to be ?
While the origin of manga dates back to as far as the 12th century. Pre-war manga magazines, like Eshinbun Nipponchi and The Japan Punch were attempted, but their sucesses were mixed, and mostly unsuccessful for a variety of reasons. The hardly resembled manga today, but mores resembled picture books of text, more synonymous with Chinese graphical art of the periods. 

The media we refer to as modern manga, was mostly  born though the commercialism of post-war Japanese economy. Western influences particularly American culture played a big role. How it all came about is due a variety of factors, but the wave of commercialism is what helped carry it around the world. 

Manga itself is an immersive social environment that encompasses various social aspects, from history, to language, to politics, religion, family, economy, and education, manga reflects Japanese society at its core, the good and the bad. The myths, traditions, beliefs, rituals, taboos, and fantasies of Japanese life are laid bare on pages of paper. The commercial proliferation of the Japanese publishing industry created a channel of expression that allowed the for the mass distribution of both real and fantastical ideas of the time. Manga as a medium is most relatable to people on a personal level because of this. While not always applicable the intimate ideas and subjects covers by the media connect to many on a personal level. While manga tends to depict a microcosm of Japanese Society. It's problem are not limited to Japanese people and society. These joys, sadness and tribulations can be share by all as fellow humans who can empathize. 

Manga is a medium of expression for people to other people. People might not see eye to eye all the time, but when they do, the connection they make go beyond the images and words on the page.