There is a good blog by Jason Teunissen about Mendix Modular Design System https://medium.com/mendix/setting-up-your-modular-design-system-in-mendix-9-6-eed2d0063411 it will tell you a lot about how this works and the routing principles. However it doesn't really cover your question of limiting styles to your specific module. The way Mendix (and scss imports work) is that it loads everything defined in the imports, so the entire project has access to all the classes defined, the order of loading and specificity still determines which styling/class is applied.
What you could do: