What are the most effective ways of using different hair serums?

I need help understanding hair serums. I have type 4 hair. I have different type 4 textures throughout my hair so saying type 4 is easier. I live in Australia therefore I do not have access to most products that are reviewed/used by popular natural hair vloggers. I DO have access to garnier/organix/tresemme and just the basic products you'd find at supermarkets. I do NOT have access to Shea moisture products, Carols daughter or any brands that are based in America. I wont go out of my way to pay postage to purchase them either, I'd prefer using easily available inexpensive products. I'd like to know if anybody uses serums from the aforementioned popular generic brands and what you've used them for? I'm not bothered about what texture you have; texture isn't important to me, products and techniques are, Thank you.

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