So after reading everyone's posts on this thread so far no one understands how these apps and sites work.
It's simple cash back, there are tons of sites and apps that do this. And if you notice almost all the stores you earn from are the same and have similar returns. Most of the cash back sites pay out in gift cards or paypal, a few do crypto which I think is great. As for "a few sats" between the my personal uses of cashback sites I've earned about 2.5k in the past 3 years not doing anything out of the normal.
So do you think these sites are paying you money for free then what is the benefit for that site? They are actually collecting the user data to target advertisements based on what they are buying more often, sooner or later you will see that kind of ads that you like to buy will popup while surfing the internet or while scrolling newsfeed, social media.
So they are targetting you and tempting you to buy a product even if you are actually no need that, you will actually spend more money but you consider you saved money by spending there.
No, so if they offer you 5% back for shopping at Amazon, they use their referral link to Amazon (Amazon's affiliate link) odds are they are getting 10% from Amazon and splitting the difference with you.
What reason would foldapp need or how much do you think they would make selling your data to google or facebook, oh wait fb/google already get your info just by having those facebook/google installed on your phone. It's also kind of hard to remove google from your android phone seeing as google owns andriod unless you use mods.
It's all affiliate links, odds are there is some data selling but not too much. At least from the limited spam I get all the spam is from things I've signed up specifically for none of it is from any of the cashback sites I've used, at least based on the email headers that are coming in.
Learn some web design, create a decent site with good content then sign up for affiliate programs with the big players, Amazon, Walmart, Bestbuy etc. you'll see that they offer you a nice amount if any of your site visitors use your links. So some web devs created sites specifically for using their links, tell your users if you make a purchase on this site you will get x, you never have to tell them you make anything but it pushes them to buy using your link.
And before anyone says anything about the apps that you upload your receipts to, yes odds are there is some analysis going on with the things you buy but a larger portion of that is the new "coupon system" now the companies that issue printed coupons are going digital, you get paid by them by proving you made a purchase and if one of the items qualifies for a coupon you get x back (usually the value of the coupon).
Edit* Also I hope none of you are using facebook or google's dns if you are worried about your data being sold.