A mirrorless camera is always building the EVF image from the sensor readout. When setting effect is off, the camera is no longer trying to show an exposure-accurate preview of your current shutter/aperture/ISO. Instead, it processes the sensor signal to make the preview easier to see.

In simple terms, the camera reads the image from the sensor, boosts/amplifies the signal, and maps it into a viewable brightness range for the EVF/LCD. It may also compress the dynamic range from the higher-bit sensor data down to the 8-bit display image. The processor then sends that adjusted image to the EVF.

So the bright finder view is essentially a processed, gain-boosted live feed from the sensor, optimized for visibility rather than for matching the final exposure. That’s why it works well in dark studios with flash, where the actual exposure won’t be represented until the strobe fires.