You can read more about it here:

Architecture and some use cases description:
https://clever-solution.com/case-studies/scalability-opporty-plasma-cash

Trilemma of Decentralization, Security and Scalability:
https://www.c-sharpcorner.com/article/plasma-cash-side-chains-for-ethereum/

Data Structures:
https://habr.com/en/post/455988/