Saving renders for compositing
It’s a good idea to separate the different steps of a pipeline for complex projects, but even in smaller jobs you don’t want to re-render everytime you open Blender.
First disable the compositor (ir you’ve been adding nodes already). Then save the render result as an OpenEXR Multilayered file. This will save all your passes and render layers. Render layers will be stored as layer groups and passes as layers inside that group (you get a dropdown in the compositor).
OpenEXR is an open image format developed by Industrial Light & Magic. It supports lossless encoding and up to 32-bit floating-point depth and is supported by many 3D applications and 2D editors, like GIMP or Krita. You can also use OpenEXR files saved by other applications like Maya in Blender.
All you have to do now is replace the Render Layers input node with an Image node, and select your exr file. I recommend you do this on a different blend file if you’re working on a complex scene or just plain heavy file to save RAM. Be careful with large images in the compositor though, it can quickly gobble up lots of ram and when you run out of RAM Blender will crash.