Comment by fsloth
Not your main question but -
” very old-school native programs that either are a loosely collected group of tools that allow you to manipulate files and pass them around”
Opencascade is a C++ library and manages it’s state in single process. Examples: https://dev.opencascade.org/doc/overview/html/samples.html
I think parent was referring to BRL-CAD, which is very much feels like a collection of cli tools ducktaped together.