The director definitely has a large influence in the overall composition of a film, but the director of photography (cinematographer) actually frames the shot, directs the g & e teams to change the lighting, directs the camera team, etc. The director will tell the DP what he wants and the DP will make show up in frame.

The director will usually never get "shitloads" of footage to deal with. The Assistant editors will sync audio and sort through dailies. The editor will screen the dailies to the director to start to understand his vision and begin cutting an assembly cut or a rough cut. The rough cut will be shown to producers, execs, etc. During these initial stages of post-production, the film is still being shot. When the film is finished production, the director (no longer needed on set) can fully collaborate with the editor to create something similar to the director's original vision. The director and editor reorganize and rearrange the film, and discover plot holes/problems. And finally comes the final cut. The final cut is the product of the assembly and the director's cut in coordination with the studio execs and producers. Here's when directors and studio's clash over creative control.

Sound is hella important and various high quality mics are used to capture sound. Sound designers and engineers are super imperative as well as cam ops, DIT's, 2nd assistant camera, art teams, boom ops, grip and electric teams, editors, assistant editors, post-production supervisors, post-production assistants, the list goes on.

Editing techniques won't make anything look better, besides maybe reframing super high res footage. Editing is cutting scenes together and arranging those scenes into a story. Good editing makes the story flow better, makes the viewer feel a certain way, and (usually) forces the viewer to not reflect on the process of filmmaking or editing. However, rules are meant to be broken.

Editing is commonly mistaken to include vfx, composition, color grading, etc. Composition and color grading from professional artists have a huge impact in the overall aesthetic of a film, TV show, commercial, etc.

Composition techniques are no secret. For example, the rule of thirds, the golden ratio, depth of field, etc. Filmmakers use and break these techniques to create visually appeasing compositions.

A lot of it has to do with money. A lot of it has to do with being creative.