Outline Image Effect for Unity
Sprite/Line/Mesh renderer support
HDR support (useful for bloom, etc...)
Alpha cutout option
Additive outline rendering option
Merged outline between renderers (or you can add outlines between different colors)
Erase option to keep outlines behind specific renderers
Per renderer color support (up to three different colors)
Option #1 (Recommended): Use OpenUPM or add the package using the Package Manager: https://github.com/cakeslice/Outline-Effect.git (Add from git URL)
Option #2: Download from the Asset Store: https://assetstore.unity.com/packages/vfx/shaders/fullscreen-camera-effects/outline-effect-78608
Option #3: Download the files and place them anywhere in the Assets folder of your Unity project
Add "Outline Effect" component to camera
Add "Outline" component to renderers