Back in the olden days lol. MS Paint and a mouse.
Then did Photoshop and a Wacom tablet for over a decade, but once I found Manga Studio later renamed to Clip Studio Paint I switched over. Photoshop was slowing me down, did not have what I needed for manga nor the illustrations look and feel I wanted. Like dragging a clunky piece of metal around. It also a RAM hog.
Switched over and Clip Studio Paint lets me quickly go from storyboard, raw, line to render. I love the asset store, much of it is free. I found brushes for sfx, motion lines, bubbles, magic, pencil, pen, guache,etc. Super easy to raw sketch.
The new versions that come out yearly with new features is a big help too for animation, manga, illustration, time-lapse, importing 3D and Photoshop files. Adding sound to the animations. Plus more.
The UI reminds me of Photoshops set up so didn't take long to figure out. Can do animation and huge full size illustration files with no lag or slowing of my computer.
Its used in the actual anime and manga industry for creating anime people watch even.
I also use it for making business logos and graphics for clients with its 'vector' tool. I just true vector with one click once imported into Illustrator.