In my first answer I have described how you can find the exact solution for the 2 zeros and the 2 pole frequencies (which are identical to the wanted break frequencies).
However, here is a good approach which should be sufficient for the shown circuit.
In principle, I follow the way as outlined already by Dave Tweed´s answer: Simplification of the circuit.
In the present case, you can create three different (simplified) circuits of first order only which easily can be analysed.
1.) For the first rising region of the transfer function the high pass part with C1 is responsible (C2 causes the falling part and can be neglected). Furthermore, for very low frequencies (including DC) the gain Ao=1+R3/R2 is assumed to be not much larger than unity which is the possible minimum.
Hence, for acceptable filtering it is assumed that R2>>R3.
As an equivalent diagram for the lower frequency range (without C2 and R2) we arrive at a circuit with only the three components R1, R3 and C1. It is a simple task to find the relevant time constants (invers to the corresponding break frequencies):
Using your indices, we thus find T2=(R1+R3)C1 and T1=R1C1.
2.) Above the frequency f1 the capacitor C1 is not effective any more (and the capacitor C2 is assumed to be not yet effective). Hence, we have a simple non-inverting amplifier with the gain (maximum of the transfer function) Amax= 1+R3/Rp with Rp=R1||R2.
3.) For rising frequencies, the low pass part with capacitor C2 becomes effective (C1 is considered as a short). Hence, the feedback path consists of R3||C2 and Rp only.
The time constant T3 (pole frequency) can be derived as T3=R3C2 and the last break frequency (zero) is determined by T4=R3C2/(1+R3/Rp).
Finally, it is to be noted, that all results are in agreement with the values given in the scetched BODE diagram. This can be verified using the well-known relations for a 20dB gain slope (as used in the graph with G1/G2=f1/f2).
Final remark: Thus, it can be concluded that the information contained in the scetched BODE diagram (break frequencies) also are only approximations.
Best Answer
The two statements need a small but important caveat.
Statement 1: Band pass filters will output the fundamental frequency of the square wave multiplied by the gain at the center frequency - provided the center frequency of the filter is the same as the fundamental and the bandwidth of the filter is narrow enough to filter out the 3rd harmonic (3 x fundamental).
If the filter has a gain or loss then this is simply applied to the amplitude of the fundamental frequency, just in the same way you would treat the signal passing through any amplifier or attenuator. Note that the harmonics don't actually disappear - they are simply reduced in amplitude.
Statement 2: Band stop filters will carry all the harmonics of the square wave other than the fundamental - provided it is a high pass filter and the cut off frequency is higher than the fundamental.
Note that a low pass filter may act like a bandpass filter if the cutoff frequency is greater than the fundamental.
Both statements also assume perfect filters with very steep cut off slopes and flat tops.