Digital Capture: Frame averaging for vastly lower noise

Frame averaging means taking more than one exposure and then a

veraging them together eg multiple exposure of the same scene. Random noise quickly disappears, leaving detail intact. The averaging can be done for RAW and/or JPEG. The camera must be motionless, and the subject ideally will be still.

best used for extreme dynamic range images
for most images, proper ETTR (Expose To The Right) exposures suffices
drastic reduction in noise of all kinds (except stuck pixels)
shoot with remote release. Can also be shot with 4-frame pixel shift.
shoot as fast as possible
electronic shutter strongly advised
do not touch the camera; all frames must expose without contact!
subject motion will blur, but that is often OK

Processing
Adobe Camera Raw + AI Denoise=10 = superb reduction of chroma noise, but not much for random luminance noise.
process to wide gamut 16-bit TIF

Processing:
1.Process images to wide gamut 16-bit TIF eg ProPhotoRGB
2. Layer all images in Photoshop eg File - Scripts - Load Files Into Stack

Blending, EITHER:
3. Use the downloadable script (best)
*or
3. set each layer's opacity to 1/N, then merge all layers

Final image is the average of all—ultra smooth! Caveats:
sensor defects can arise eg banding
This process ought to be automated, producing a single RAW file, but camera vendors have largely not bothered, PhaseOne excepted.
Certain DSLRs like Nikon D850, but it had to be done in camera manually after taking frames.