In the Bakuman manga, the authors show that the manga artists create a manga without using any technology like computers. Is that how the mangas are drawn?
Every artist has different drawing techniques.
- Some fully use the computer (using a drawing tablet and photoshop)
- Some fully draw by hand (using pencil, then pen, then erasing the pencil, the making the pen lined bolder)
- Some use a combination of the two (Draw by hand, scan to computer, complete with photoshop).
It's really up to the artist.
Just to add up to @Madara's answer.
Computers are also used to generate backgrounds. They'll generate wire mesh objects and room, if neccesary they'll apply complicated lighting and shadows (like sunlight passing through a complicated ceiling), apply textures to the models, and then print them and work by hand over them!
You can see the process of an artist mastering this tools in the omakes from Ken Akamatsu's Negima! volumes.
He would sometimes take photographs in many places or search for pictures of certain architectural designs (castle, palaces, etc) and he and his team would reproduce them on wire mesh models.
The work of Ken Akamatsu on sceneries is gorgeous, and most of the time you wouldn't think was done with CGI since it still has a lot of hand-drawn work.
He also creates creatures models to create and reproduce more easily crowds.