I have a tiny problem with understanding a thing here.
for the 1st the phase shift is 0 degrees, so on its modulated wave we put 180-degree phase shift.
for the 2nd 0, the oscillator puts a 90-degree phase shift in the carrier signal and as it's a 0-degree we have to put 180-degree shift.
When we sum up the two 0's in the final wave, how are we getting the -135 degree???
I'm unable to understand the summing. https://prnt.sc/gP0OypJ3n_jB
Your constellation points are the sum of {+1, -1} and {+1j, -1j}, so you get the points {+1+1j, +1-1j, -1+1j, -1-1j}, which have these odd multiples of 45° as angles.
That's one way to look at it.
The other is to realize that if you had only multiples of 90° as angles, only the inphase- or quadrature component can be active at a time, as the amplitude of the other must be 0. But in your schematic, you see that both are active with a non-zero amplitude at the same time.
In the end, it really doesn't matter. The receiver gets the signal with a different, arbitrary phase, anyways, and will rotate it however it sees fit for best decidability. So, maybe it is assumed that the 90° point on transmit side gets simply rotated to be 135° by the receiver (just as any other points are rotated by the same amount).