Understanding analog frontend for PIR sensor
The op-amp, with the aid of the external parts, acts as a bandpass filter and high-gain AC amplifier, with clamping.
The diodes lower the gain for large (over 500mV-ish) output to about 1.
AC gain in mid-frequency band is approximately 1+R13/R11 = 221.
C34 and R13 set the high frequency cutoff at 1/2\$\pi\$fC =  10.6Hz.
R11 and C33 set the lower frequency cutoff similarly at  0.7Hz
This frequency band (0.7 to 10.6Hz) is related to how fast we are expecting a warm object to be moving across the facets of Fresnel lens of the detector.
The detector itself is an AC device, incidentally, and will have its own upper and lower cutoff frequencies and an output bias voltage that may change with ambient lighting and supply voltage. The output is typically the source of a JFET follower. We also want to reject 100/120Hz noise from mains-powered ambient lighting.