How can I learn all the knowelege like you did?
Through years of collecting, handling and researching these awesome firearms.
I can tell just by looking at the gun, based on its features, that it is a "pre-war" or early "wartime" (WW1) C96. Those are collector terms used to refer to certain features, some are subtle, some are very distinct. Examples include machining, hammer style, receiver cuts, safety (hollow, filled), etc.
A good reference book is "Broomhandle Pistol 1896-1936" by Erickson and Pate, but it is quite expensive and difficult to find.
Otherwise, there is a decent amount of information online, but I find most of the advanced info can only be found in print.
As for this particular C96 being used during WW1, the "1920" marking is a giveaway - more on that below. While the Germans did not use 7.63 C96's during WW1 "officially" (i.e.; actually "adopted"), tens of thousands were purchased by the German's (all states, and by the Austro-Hungarians) for use during WW1. Many saw front-line combat.
The "1920" marking indicates that it was in German inventory, specifically during the Weimar Republic, some time after 1919. Note that the "1920" marking was not necessarily applied in 1920, contrary to much info online. It could have been applied anytime during that era, as it was a necessary marking for "triple entente" / "allied" inspectors, to ensure Germany was complying with the "Treaty of Versailles".
The marking indicates that it was an "official" small arm, and allowed within Germany's post war arms restrictions.
Here is the interesting part - it still has its longer original length barrel. Germany cut most down under 4" to comply with treaty terms. However, some pistols were allowed to remain "long", mostly for Police use.
With all of this evidence, we can surmise that it was manufactured during, or just prior to WW1, was acquired by the German military, saw service during WW1, was retained post war, and kept in Weimar inventory.
Everything past that is speculation, but based on the condition of the gun, and the typical deep Chinese bluing, I suspect it was re-done / refurbished by a Chinese arsenal. How did it get to China? Likely as aid from Germany. From the mid 1920's until 1941, Germany was modernizing China both militarily and industrially. This included a lot of material aid in the form of small arms, vehicles, helmets, etc. Many German officers also trained the Chinese officer corp, etc. This greatly helped the Chinese resist the Japanese, ironically.
Many Chinese C96's were imported into Canada and the USA in the late 1980's and early 1990's. I suspect this is one of those pistols.
Hope this helps!