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): Download from the Asset Store: https://assetstore.unity.com/packages/vfx/shaders/fullscreen-camera-effects/outline-effect-78608
Option #2: Use OpenUPM: https://openupm.com/packages/com.cakeslice.outline-effect/
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