Hi PG,
When they say 3db less that means 3db less than the midrange frequency at the corner. They dont draw it like that because it's an asymptotic Bode plot and that is a simplified way of drawing these curves. It's drawn with a corner rather than a curve. The true response is a curve which is at 103db at 10Hz.
Plots like this are often drawn with straight lines when they are really curves, but the straight lines show the major part of the response curve. The 3db down points are drawn as corners. Sometimes these straight line plots are grossly inaccurate and have to be corrected using special curves depending on the damping ratios, but when the response is not too wild straight lines tell the story quite well but without being perfect at the corner frequencies.
In drawing these plots a pole causes a relative slant downward and a zero causes the opposite (relative to what came before it), so after a pole a zero will create a horizontal line again because the zero shifts the slant exactly opposite to what the pole causes so they cancel out and what remains is a horizontal line. Using this simple guideline we can draw these plots by hand knowing only the poles and zeros and WITHOUT a computer or calculator of any kind. (Note: A better way of looking at this is that a pole causes a decrease in slope and a zero causes an increase in slope).
Op amps are DC amplifiers, and DC amplifiers can amplify both AC and DC.
AC amplifiers can only amplify AC.
Decade is a 10 fold increase or decrease in frequency, so 10Hz is a decade lower than 100Hz.
Octave is a 2 fold increase or decrease, so 10Hz is an octave lower than 20Hz.
For a plot that goes -20db per decade, that means that after the corner frequency it decreases an additional 20db for every decade, so a signal that is 0db at 10Hz would be -20db at 100Hz and -40db at 1000Hz.
For a plot that goes -6db per octave if the signal is at 0db at 10Hz it would be -6db at 20Hz and -12db at 40Hz and -18db at 80Hz (an additional -6db every time the frequency doubles).