reduce the brightness of an hdri background

 Turn down camera exposure / settings, adjust strength of background image

Cycles render engine - Outdoor HDRI too bright - Blender Stack Exchange

https://blender.stackexchange.com/questions/69605/how-can-i-create-a-silhouette-using-hdr-in-cycles/69615#69615

Can use two nodes for background also, one to light the scene and one for the camera to see.

https://blender.stackexchange.com/questions/113908/increase-brightness-of-hdr-to-light-the-scene-but-keeping-the-background-at-a-l

Explanation of "factor" at

Make realistic environment lighting in Blender (in 10 mins) (youtube.com)