I know little about the specifics of farm machinery and it might very well work with schematics etc. so that stuff can be fixed by anyone reasonably skilled (and having access to the right parts and tools).
However, for some machinery (might not be in farming) it will need real training so even with schematics, docs etc. we do not want untrained people to fix it. These people might not work for the manufacturer, but repairing is "gated" in a sense (repairing commercial aircraft, for example). As far as onsite vs. shipping is concerned, I think with really heavy equipment most maintenance will be onsite as shipping makes no sense at some point/transport is next to impossible.
However, for some machinery (might not be in farming) it will need real training so even with schematics, docs etc. we do not want untrained people to fix it. These people might not work for the manufacturer, but repairing is "gated" in a sense (repairing commercial aircraft, for example). As far as onsite vs. shipping is concerned, I think with really heavy equipment most maintenance will be onsite as shipping makes no sense at some point/transport is next to impossible.