Well, It all happens systematically.
Nowadays many payment gateway platforms are available which act as an intermediate between buyer and seller. When buyer buy's a product, the payment gateway companies provide various options to buyers to pay for its purchase. This all money collect through gateway is transfer to the seller.
If buyer refuse to take the product then buyer initiate a return request. At the time of return request seller ask for their payment details to refund their money. After returning the product seller transfer his money to buyers account.
And this answer is for how do you can gain money online.
There is a online gaming app called "Rummy Gold" which is currently providing very good offer on their platform. You can gain money by playing games on this platform. Go through this blog for complete details -
https://rummygoldapp.com/rummy-gold-apk-download-get-rs-41-sign-up-bonus/