As you probably know, Mitsubishi is one of the largest conglomerate companies with many, many divisions in numerous market segments making all sorts of products.
The engine from what I've seen/read is primarily Mitsu with a smattering of parts from other common Japanese OEM manufacturers. The transmission is (& possibly also the xcase) made by Aisin. From what we can tell the axles/ring and pinion gears are made by Mitsu since they do have a large heavy industries group of companies unlike other car manufacturers who rely on 3rd party vendors (dana, etc.). For example the adjustable shocks in the gen II for example are made by KYB which from what I can recall is actually part owned or possibly even fully owned by Mitsubishi.
That said, I don't believe any of the Montero's share powertrain parts with others unless those parts were OEM'd by other manufacturers.
Are they interchangeable, with OEM'd products yes to a degree. For parts that are Jeep, Chevy, Ford, etc. usually not from what I've seen/heard.
Others on this forum (Justice for example) are also very knowledgeable on the Montero so hopefully they chime in. Additionally, 4x4wire has a breadth of knowledgeable forum members as well.