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minecraft:memory_sensor.in_range_of_entity is an entity component: it goes in the components object of a custom mob, or in one of its component groups. A sensor that lets the entity remember when other entities come within range. It has little public documentation, so test it carefully.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:memory_sensor.in_range_of_entity": {}
}