It would be great if you could add some pictures showing the results you’re looking for, as not all of us have too much prior experience with LaTeX! 
With that said, for lines specifically, I wonder if you might be looking for Bezier curves. In CeTZ, you can follow this tutorial: A Picture for Karl's New Students | CeTZ Documentation
Though this is also supported natively in Typst through curve: Curve Function – Typst Documentation
Which allows you to implement very complex paths.
For rectangles, though, you can use a rect with radius: Rectangle Function – Typst Documentation