The .sma file is the source code, it is used to build the .amxx (binary) file, and it is not needed on the server to run the plugin.
I learned a lot by looking at other plugins and learning about mapping resources. dodplugins.net had a resource called DOD VERC which has map entities and I have backed up
here. The "weapon_" entities are map entities that are listed in there. It might not be entirely complete but it's served me well.
Most concepts in DOD are the same as in other mods like Counter-Strike 1.6. Player models should be handled in the same way and there are player model plugins for swapping out player models. Many of these plugins I dislike (I really like a mostly vanilla DOD) so I won't have much experience with them.