The developers of a blockchain can choose to support the Ethereum Virtual Machine (EVM) to leverage the available development technology stack and hence the Ethereum developer community. Otherwise they would have to build this technology stack for their blockchain.
Each blockchain uses a consensus algorithm that they think is the best compromise for the scalability trilemma (decentralization, scalability and security)
Ethereum 1.x uses Proof of Work. Ethereum 2.0 uses Proof of Stake.
Harmony use a version of Proof of Stake.
To learn more see The crypto in cryptocurrencies (and beyond) posts on Proof of work and Proof of stake