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minecraft:bounciness is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Controls how much the entity bounces back after hitting the ground or a wall. Higher values give bouncier movement, 0 means no bounce.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:bounciness": {}
}