A frequency response is read on a logarithmic frequency axis, because what matters about it happens per decade rather than per hertz. Drawn linearly, the first decade of a four-decade sweep is the leftmost thousandth of the chart and everything below the corner is a vertical line at the origin.
axis x log says so — and it changes the sampling as well as the drawing. A fixed number of samples spaced linearly across 10 Hz to 100 kHz puts four of them below 1 kHz; spaced logarithmically, every decade gets the same number, which is what a reader of the chart assumes has happened.
axis x "Frequency" says what the axis measures. The unit is drawn from the dimension either way; this is the quantity beside it, and a chart that names both is one a reader can take out of the worksheet.
One resistor, one capacitor, and the corner frequency where the reactance of the capacitor equals the resistance.
The magnitude response, as a ratio and in decibels. Written as functions of a dimensionless frequency in hertz so that the plot can sample them: a plotted function takes one number and returns one number.
At the corner the magnitude is 1/√2, which is where the decibel figure of −3 dB comes from — it is not a convention but the value of the function.
In decibels the vertical axis is already logarithmic — a decibel is ten times a logarithm — so it is drawn linearly and the curve is the familiar straight asymptote falling at 20 dB per decade.
As a plain ratio the same response needs a logarithmic vertical axis to show the same straight line. The two charts carry the same numbers and the same shape, which is the point: a decibel is a unit for reading a log axis without drawing one.
Back to a linear vertical axis, and a window on it: the phase of a first-order response runs from 0° to −90° and nowhere else, so the axis is told to show exactly that rather than fitting itself to the sampled extremes.
axis chooses what is drawn. The span in the plot call chooses what is computed, and the two are kept apart on purpose: a window that quietly resampled the function would make zooming a chart change the curve.