does ethereum support multiple concensus methods?

Hi, I'm starting on reading about Blockhains and their applications.

Two papers I've read claimed that they used a private/permissioned blockchain that uses a proof of authority concensus mechanism. However, they also mention that they're using Ethereum smart contracts and as far as I know, Ethereum is a proof-of-work public blockchain.

My questions are: 1) How is this possible? Did they fork Ethereum and modified the source? 2) If Ethereum somehow supports this, is this possible with all blockchains that support Smart contracts, say Cardano?

Papers I've read: Gürsoy, G., Brannon, C.M. & Gerstein, M. Using Ethereum blockchain to store and query pharmacogenomics data via smart contracts. BMC Med Genomics 13, 74 (2020). https://doi.org/10.1186/s12920-020-00732-x

Aicha Fatrah, Said El Kafhali, Abdelkrim Haqiq, and Khaled Salah. 2019. Proof of Concept Blockchain-based Voting System. In Proceedings of the 4th International Conference on Big Data and Internet of Things (BDIoT'19). Association for Computing Machinery, New York, NY, USA, Article 31, 1–5. https://doi.org/10.1145/3372938.3372969