Hi there! I am fairly new to ZBrush, and I have no idea how to merge multiple subtools/ZBR files. Could someone explain this process to me in an easy way? BTW I have no prior experience with digital sculpting/ modelling. (I don’t know if that will come in handy for you, but basically I have no idea what mesh, polygroups, etc. mean) Please help, thank you 
Simple answer? There are multiple options for merging geometry in the subtool palette. Merge down, merge similar etc. Load a tool, append it into the subtool palette of another tool, and pick whichever is most appropriate. Each has a tool tip explaining their use.
The good answer? Your inquiry is broad, requiring knowledge of many different parts of Zbrush to know what is the best solution for the specific situation you have. Do you want to merge a subtool into another, simply carry around multiple meshes in the same file, or do you want to combine meshes into new contiguous geometry? Do you want to be able to easily isolate different parts of a single subtool?
The best thing to do would be to go here:
And use the search function to study the the different 3d mesh types like Polymesh 3d, the Tool Palette, difference between Polygroups and Subtools, how to Append tools into the Subtool Palette, or Insert Meshes, Merging subtools, Remeshing, and Dynamesh. There is no easy answer without understanding Zbrush fundamentals first.