As you say, the issue of moving the wounded was a major logistical operation and survival of the those men wounded depended largely on how well that operation was performed.

When we talk of the wounded, we are generally speaking of two groups, the Walking Wounded and the Stretcher Cases*. As the name suggests, Walking Wounded refers to those men who have suffered an injury of some kind but are still capable of moving under their own power or with minimal assistance from someone in a similar condition. An example of this might be someone who has received a bullet wound to the arm so can still travel the distance back to his lines and to the dressing station without assistance. Stretcher cases, again quite obviously, are those cases in which a wounded man requires significant assistance to reach safety. This may include a soldier who has lost a leg, suffered and traumatic brain injury or an abdominal wound for example. These distinctions were not always clear cut. In the Official History of the Australian Army Medical Services in the War of 1914-1918 it is stated that:

If there is any doubt whatever about a man being able to walk, make his a stretcher case. His chances of ultimate recovery will undoubtedly be increased thereby. Where the number of casualties, however, is in excess of the stretcher-bearers available to carry them out, this principle does not hold. Obviously the thing to do is to get them out of the Forward Area, the zone of greatest danger, as soon as possible. Every effort should be made to avoid congestion of wounded in the Forward Area, even if extra pain and discomfort are occasioned to men by making them walk a few extra miles until transport is obtainable.

This might seem somewhat cruel but the priority was to get the wounded out of the forward trenches so that fresh troops could occupy them for one and to also remove the wounded from any further potential danger such as enemy artillery or counterattack. Having wounded men remain in the Forward Area awaiting available stretcher bearers when there was the possibility that they could walk out unassisted or with minimal assistance only created issues.

In the Australian divisions, stretcher-bearers were primarily made up of bandsmen until mid-1916 when they were replaced with men specially selected and trained. Generally these were larger men who could bear the weight of the wounded and they were instructed in basic first aid to render immediate assistance to the wounded. That first aid could consist of splinting a broken limb or applying a tourniquet. Most battalions had 32 dedicated stretcher bearers though during larger battles, men would be drawn from the combat strength to supplement.

The wounded would either walk or be conducted by stretcher bearer to the Regimental Aid Post (RAP) located at a central position just behind the regiment's front line. It may be also that Stretcher Bearers relayed wounded between certain posts rather then carrying them the entire way. The RAP was responsible for providing immediate wound treatment to stabilise a patient before they moved further along the chain. Stabilisation generally meant preventing the wounded man from going into shock and could mean the application of Morphine or a Tourniquet. If either were applied, an M or a T were marked on the patient's forehead. Walking Wounded would also be have been assessed here and if possible they would be treated and returned to their unit depending on the severity of their wound or directed along with the Stretcher Cases to the Field Ambulances for transportation to a unit better equipped to treat them further. It is likely that the collection of tents that seen at the end of the film 1917 represented the Regimental Aid Post of the 2nd Devons.

From the RAP, wounded would be conveyed to a designated field ambulance Advanced Dressing Station (ADS). Further assessment would take place with special attention given to those marked with an M or a T. Urgent cases would be immediately moved to the Main Dressing Station (MDS) while less serious were transferred onto a Casualty Clearing Station (CCS) Walking Wounded would be directed and guided to an Advanced Dressing Station for further treatment and assessment. If further treatment was required, groups of Walking Wounded would be conveyed by transport to a (CCS) via wheeled transport. The MDS was the first major installation along the Medical Evacuation chain and it was at this point that surgical procedures were undertaken. Gas victims were also sent here before being transported along the chain.

The CCS was a distribution point from which wounded would be triaged and moved onto base hospitals but could also be provided with treatment.. The role of the CCS changed quite drastically over the course of the war and it is difficult to generalise the role of the unit. In the British Army, CCS were initially intended to fulfill the role of a sorting center, wounded would arrive and be sent out to different hospitals depending on their wounds and prognoses. By the middle of 1915, CCS' has evolved to handle surgical work, head injuries and the treatment of compound fractures. Assessment of postmortem results concluded that many deaths from abdominal wounds were the result of hemorrhaging. Surgical treatment of such wounds were supposed to take place beyond the CCS but due to the time sensitivity of those wounds, CCS became, in 1916, the primary center for the surgical operations needed to save those men's lives. This also resulted in the CCS' being moved closer to the front to enable the wounded to receive treatment sooner. By 1917, the Field Ambulances had taken over the role initially intended for the CCS and they began allocating the wounded to the necessary treatment center. CCS' adopted specialty roles with different stations treating different types of wounds such as gas victims, abdominal wounds, head injuries etc.

From the Casualty Clearing Station, wounded (but now treated) soldiers would be transported by rail or by motor transport to hospitals where they may have undertaken further treatment depending on their wound and would start the recovery process. Once a soldier was well enough to be discharged from the hospital, he would be sent to a Command Depot or Convalescent Home where he would spend time recovering further before being returned to his unit. If his wounds were too severe and he could no longer undertake active service, he would be transported to the coast (if he was not French), transferred to a hospital ship and then transported back to England for further processing and eventual discharge.

This diagram gives you a rough idea of the Medical Evacuation Chain

The above is an ideal system but there were always exceptions, mistakes, etc that led to men undertaking different journeys then the one I have described.

Sources:

Official History of the Australian Army Medical Services, 1914–1918 by A. G. Butler

The Chain of Evacuation of The Royal Army Medical Corps

The Great War and the R.A.M.C. by F. S. Brereton