Symmetric

1. Pre-select a line, or linear edge.
2. Select two other geometries (of similar type to each other).
3. Click .

Toggle on constraints while you make selections. Each pair of entities selected are constrained to each other. Click to toggle off, or select another tool to toggle off automatically.

Symmetric only constrains the underlying curve to be symmetric. For example, when applying the Symmetric constraint to two arcs, the underlying circles are made symmetric, but not the end points. You would need to add those separately and/or drag them closer to what is needed.