How does the 12 keyword back phrase works and how does it allow the creation of infinite number wallet address across multiple cryptocoins?
Per BIP39 the original entropy in form of mnemonic can be translated to master seed per BIP32. 


  This document describes hierarchical deterministic wallets (or "HD
  Wallets"): wallets which can be shared partially or entirely with
  different systems, each with or without the ability to spend coins.
  
  The specification is intended to set a standard for deterministic
  wallets that can be interchanged between different clients. Although
  the wallets described here have many features, not all are required by
  supporting clients. 


Using BIP44 one can use different coin_type codes for different currencies and standard derivation is m / 44' / 0' / 0' 

BIP44:


  m / purpose' / coin_type' / account' / change / address_index


As far as I know BitPay/Copay use this derivation type.