There is no way to get free Unknown cash (UC) in PUBG mobile. You cannot get unlimited UC in PUBG mobile. You need to buy UC from with money in the following manner.
1) You can back UC from Paytm, Phone Pay and other apps (You can use the cashback money that you got from these wallets)
2) You can purchase UC from PUBG mobile shop
3) Buy an Elite Pass or Royal Pass. So, every time you complete a seasonal mission. You will win different types of prizes and rewards one of them is UC
4) Use Google Opinion Rewards app. This app, designed by Google, gives the user surveys that they must complete in exchange for money.
Please don’t try to use any illegal methods you might be banned from the game.