天然 (tiān rán) describes something that is natural in the sense of being unprocessed or formed by nature, not made by humans — it's used for physical things like ingredients, resources, or landscapes. 自然 (zì rán) is broader: as an adjective it means 'natural, unaffected' (e.g., a person's manner), and it also functions as a noun ('nature') and adverb ('of course'). 天生 (tiān shēng) and 先天 (xiān tiān) both refer to inborn traits, not physical objects.