Wechat pay in China is interesting. It costs nothing to add money to your balance from your bank, or to pay someone from your balance. It only costs the end merchant who wants to withdraw it from their balance back into their bank. If they can keep it in Wechat pay and spend it on other things (which is very easy as it and Alipay are the primary payment methods for everything), then there's no charge.

I guess Tencent are making their profit from the interest they earn on the money that was transferred into them that just stays in people's Wechat wallets in effectively a parallel currency.

Well, that definitely creates a powerful pull from customers to make vendors accept Wechatpay for transactions.

And no doubt they’ll find a way to spend it in the app considering you can manage almost all aspects of your life within it.

Revolut works similarly. You don’t pay any fees on transfers to other Revolut accounts, but you do for other bank accounts.