How to build corner with two intersecting line segments?

Questions
1

how to build corner with two intersecting line segments?

I wan to join these point such like miter or round?

11.PNG
11.PNG565×1270 51.3 KB

12
12850×700 17.1 KB

2

See Construction of frames with contour/profile

and from the Collection of examples from discourse.threejs.org

ProfiledContourGeometryMM

ProfiledContourGeometryUVs

3

I found this