App like boost gives cashback for every transaction we make, n this is on top of cc benefits that we can get

Do they pay merchants some % of total sales to partner with them, or the other way round, where merchants share some % of their sales with e-wallet company?

While other one like Grab Pay could leverage their food delivery services to get food merchants on board, but after all, is it sustainable with all the promos n cash back?

or they are all bleeding till a point that they are powerful to sell or make use of these huge pool of customer n transaction data they collect?

inb4 money laundering lettew

They just bleeding money to lock you into their ecosystem... once they have sizeable userbase, all the cashback, etc will be lowered. Look at Lazada now. Before this, they used to give a lot of vouchers/promo. But now barely any promo left.