> I thought you need to clean and full redraw if anything changs anyways?

Unless you use an ancient teletype, you don't have to redraw everything. That would make any interactive applications way too slow/flickery.

You can move the cursor arbitrarily in the terminal and start overwriting characters from there. So you need to track state to know what is "dirty" and needs refreshing. Occasionally you issue a full redraw to catch missed artifacts left behind or when the terminal is resized (SIGWINCH).