Your eyes are the supply. The demand (customer) is people wanting your eyes. Relevance in ads is if your eyes are a candidate to at some point buy their product.
What your taking about at costco is essentially house ads or really signage.
Remember unless you are a brand you are not the customer for ads.
But the real customer, the brand, cares if the ad is relevant to you because they are either paying for impressions or traffic. Either way, if the ad is not relevant the analytics tools will show that campaigns in Google perform worse than campaigns bought in, say, Meta or TikTok.
Your eyes are the supply. The demand (customer) is people wanting your eyes. Relevance in ads is if your eyes are a candidate to at some point buy their product.
What your taking about at costco is essentially house ads or really signage.
Remember unless you are a brand you are not the customer for ads.