Unity object outlines

Built-in render pipeline:

The effect requires the post-processing stack (v2) package. To add the effect, click Add effect... on a Post-Process Volume component and select Custom > Outline, or add a OutlineQuickVolume component on an empty game object. All the objects with a specific tag (default tag is "Outline") will have the effect applied;

Universal render pipeline:

To add the effect, click Add Render Feature and select Outline Feature on the Forward Renderer Data asset. All the objects on a specific layers (chosen in the outline feature settings) will have the effect applied.

Limitations:

All the objects are drawn into a single buffer, as a result all the properties of the outline are global. This means that you cannot have objects with different color, size outlines at the same time.

For outlines with different properties multiple buffers can be drawn, or in the case of Universal render pipeline, multiple passes can be added, one for each set of properties.