How is signal to noise ratio actually measured by receiver equipment?
The SNR is simply the mean-square of a demodulated symbol divided by the variance of the signal, or in dB:
$$\DeclareMathOperator{\SNR}{SNR}\SNR =10\log_{10}(\mu^2/\sigma^2)$$
$$=20\log_{10}(\mu/\sigma)$$
A typical metric for this in receiver equipment is the Error Vector Magnitude (EVM) where an &quot;error vector&quot; is the Euclidean distance from the actual sample at the optimum timing location in each symbol to the actual symbol location in a reference waveform (as the distance to closest decision boundary, just prior to decision). EVM is the normalized rms of the error vectors over multiple samples, where the waveform and the decision boundaries are scaled to either the rms amplitude or a maximum amplitude in the constellation, depending on which standard is used. We can therefore use one sample for every symbol to compute a sufficient number of errors to derive a statistical quantity.

See this post for more details on computing EVM from which the above graphic was copied: How to calculate EVM in %age of an Equalized Constellation in 16QAM?, but to describe it simply we statistically estimate with enough samples the standard deviation of this normalized error. The EVM is the average of the noise magnitudes as a percentage of the normalized constellation which represents the signal. (The average of the magnitudes is not the same as the magnitude of the average ($\mu$) so an additional small adjustment factor for this is needed in translating EVM to SNR that depends on the signal constellation used and the normalization that was used for computing EVM).
The relationship between EVM and SNR is therefore:
$$\SNR =  20\log_{10}(1/\text{EVM}) - K$$
Where $K$ is an adjustment factor depending on the peak-to-average of the constellation and how the constellation was normalized to the peak for computing EVM.
This would be affected by the portion of the noise as in the OP's figure that is within the bandwidth of the receiver after demodulation just prior to symbol decision (which is the noise bandwidth we are concerned with: the waveform bandwidth).