Why do some phone brands (Poco, Tecno, Realme) cost so much less than Samsung or Apple?

I own a Poco F7 and Tecno phone—both perform great for their price. But they cost 40-50% less than comparable Samsung or iPhone models. Is it just brand premium, or are there actual quality/component differences? Are budget brands cutting corners somewhere I'm not seeing, or is it purely marketing and profit margins?