To make sketch automatically snap to 3D element at background, go to Tools > Options > Mechanical Design > Sketcher > Constraint > SmartPick dialog box.
Check Background Element
For example, move mouse cursor to detect solid edged & coincident constrain automatically create during line creation.
Tangent constrain can be automatically create after setting the auto snap. Sketch element also can be drag to 3D element to define new constrain.
