There is a specification of the protocol, which is open source: https://github.com/ethereum/consensus-specs

And there are over 5 interoperable, independent and open source implementations of said specification:

- https://github.com/sigp/lighthouse/

- https://github.com/ChainSafe/lodestar

- https://github.com/status-im/nimbus-eth2

- https://github.com/prysmaticlabs/prysm

- https://github.com/ConsenSys/teku

- https://github.com/sifraitech/grandine

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