I think the documentation is available for developers but what about
the "common man" who wants to be more familiar with the system?
When installling programs in Amiga I know exactly where the files go
(thanks for the well organised os and it's documentation) but in
PC I don't...
There's literally a shyt-ton of information out there. Windows internals books, the unwritten Windows manuals, Windows Bibles, etc. All have been available in bargain bins and regular computer shelves at the book stores for years. (Though these days the book "store" ain't quite what it used to be.) Commercial operating systems generally don't come with the printed manual we got with GEOS (though I recall a good bit of griping that GEOS was too closed at the time) and printer manuals don't come with a chapter dedicated to how to create graphics with ASCII codes. Though hobbyist computing exists, general computing is not hobby- and tinkerer-centric but geared for people who don't care what's under the hood.
I suppose to put it another way, the majority of technically-savvy and technically-minded computer users/owners has declined past the point of being worth it to manufacturers to include extremely technical information with the products. That's where hobbyist communities such as our own come into play, and it's no use bemoaning how Xyz Commercial Vendor, Inc. doesn't give all its buyers in-depth technical details.
