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.
To create a Coincident constraint:
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.
Learn more by watching the video below:
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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.