The reason to have a "construction kit" for a given thing is that you are constructing a thing involving a huge amount of choices.
A programming language, an operating system and a database management system are all examples of complex, high level things involving such plethora of choices.
Programming languages have a variety of construction kits of various sorts, from Bison and Yacc to regular compilers and so-forth.
I hadn't heard of any equivalent kit for OSes or database system. Unfortunately, what's described gives the impression of producing a rudimentary system based on a few canned choices. I mean, fully producing an OS would involves generating calling conventions, device driver conventions, process convention, memory allocation methods, etc. (I'm far from an expert on this btw).
Sure, you can say "but all that's just done for you". It probably is done for you ... one particular way. But as far I know, the reason to produce a new OS is because you have some opinion how this stuff should work, usually because you're actually producing an OS to be use by some device and that device some needs and weirdness that needs to be taken into account.
Edit: Oh wait, just remembered, people are now creating "OSes" that are just app-containers intended to run AWS. If that's what you need, maybe this works.
Bit of a shame an "operating system construction kit" cannot be developed without using Windows.
Visual Basic? C# and the .net runtime?
"Operating System Legos"
Ouch. Please change that to "Operating System LEGO" which is more correct.
https://cosmos.network/ The Internet of Blockchains called; they'd like their name back.