In case of direct2d, there is no source code, but since it uses d3d10/11 under the hood, it's easy enough to see what it does behind the scenes with Renderdoc.
Basically d2d tends to have a policy to minimize draw calls by trying to fit any geometry type into a single buffer, versus skia which has some dedicated shader sets depending on the shape type.
So for example, if you draw a bezier path, Skia will try to use tesselation shader if possible (which will need a new draw call if the previous element you were rendering was a rectangle), since you change pipeline state.
D2D, on the other side, tends to tesselate on the cpu, and push to some vertexbuffer, and switches draw call only if you change brush type (if you change from one solid color brush to another it can keep the same shaders, so it doesn't switch), or when the buffer is full, or if you switch from shape to text (since it then needs to send texture atlases).
Please note that when tessellating bezier path D2D does a very great work at making the resulting geometry non self intersecting (so alpha blending works properly even on some complex self intersecting path).
In case on rounded rectangle, it does the same, just tessellates into triangles.
This allows it to minimize draw calls to a good extent, as well as allowing anti alias on a non msaa surface (this is done at mesh level, with some small triangles with alpha). The downside of it is that it doesn't use much hardware feature, and geometry emitted can be quite high, even for seemingly simple shapes).
Since d2d prefers to use triangle strips instead or triangle list, it can do some really funny things when drawing a simple list of triangles.
For text, d2d use instancing and draws one instanced quad per character, it is also good at batching those, so if you call some draw text functions several times in a row, it will try to merge this into a single call as well.
