Q. a local mac repair shop installed "new OSX" on a laptop by request .. but it is version 10.12.6 (?) ..
Later, when downloading several common desktop applications, upon opening them.. it says "this software requires v10.13 or later" .. I assume this is completely on purpose to get the legions of happy Mac owners off of their stable OS and into the upgrade churn ? I assume (US here) you have to have to buy OSX 10.13, register an ID with Apple, to get new software (according to them) ?
macOS upgrades have been free since OS X Mavericks[0] (10.9, released in 2013). macOS 10.13 (High Sierra) came out in 2017, so is a 5 year old OS at this point[1]. It also supports all Macs that macOS 10.12 (Sierra) supports[1]. And you don't need an Apple ID for anything other than Apple services and the Mac App Store. OS Upgrades don't need an Apple ID (even on older versions of macOS where they're installed through the Mac App Store), and you can even network boot a recovery image and install the latest version over the network[2] (Intel Macs only).
I'm a bit confused by the question and might be misunderstanding but there is no need to buy OS X upgrades, they're free. But you do need to register with Apple to get them.
You’re not entirely wrong; you do need “Xcode Command Line Tools”. Thankfully, that’s a much smaller bundle than Xcode. It contains a bunch of tools for building code, such as llvm, ld, make, and git.
Later, when downloading several common desktop applications, upon opening them.. it says "this software requires v10.13 or later" .. I assume this is completely on purpose to get the legions of happy Mac owners off of their stable OS and into the upgrade churn ? I assume (US here) you have to have to buy OSX 10.13, register an ID with Apple, to get new software (according to them) ?