When geometric elements are coincident, it means they are overlapping. You can use the Coincident constraint to make sketch endpoints coincident with other elements to make them connected.
On the same sketch plane, select an endpoint and another element.
Select Coincident from the constraints menu or drag an endpoint onto the element you want to connect to. Note: Endpoints can be made Coincident to other endpoints, lines, curves, and edges.
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Endpoints that are connected to elements with a Coincident constraint are shown with a filled center. For more information, visit States of sketch points.