Beginners quastion: How to stop drizzle and rain triggering recording?

This is exactly what the AI is designed for. If you are only interested in detecting humans, then set the AI Human threshold slider(s) for Motion Capture and/or Actions under Preferences > Cameras > Triggers to around 85.

This should cut down the vast majority of false-positive detections (though you still may get a few through on a rainy night because no algorithm is 100% effective).

Another option to improve things further is to turn off the camera's built-in infrared LEDs, and instead install a separate infrared LED illuminator away from the camera. Built-in LEDs have the effect of brightly illuminating moving objects right in front of the camera (insects, rain, snow) and this can be a major source of false positive triggers.

For the AI sliders I would recommend a setting of 85. Lower and you will get more false positives (non-events that trigger recording); higher and you will get more false negatives (real events that don't trigger recording). So you may like to set a bit higher or lower than 85 depending on your personal preference here.

Yes, the sensitivity setting determines how many events get sent to the AI for classification in the first place. I would recommend using 50 for this. The way the motion detection works is explained in the manual under Preferences > Cameras > Triggers.