A great multitude of developers and enthusiasts belonging or contributing to diverse teams spread across the world: developing, debating, and collaborating for years to arrive at the big event.

And it's all been done very much in the public view:

https://weekinethereumnews.com/

https://hackmd.io/@benjaminion/eth2_news

https://github.com/ethereum/pm

https://www.youtube.com/c/EthereumFoundation/videos

Consensus Clients:

https://github.com/sigp/lighthouse#readme

https://github.com/ChainSafe/lodestar#readme

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

https://github.com/prysmaticlabs/prysm#readme

https://github.com/ConsenSys/teku#readme

Execution Clients:

https://github.com/akula-bft/akula#readme

https://github.com/hyperledger/besu#readme

https://github.com/ledgerwatch/erigon#readme

https://github.com/ethereum/go-ethereum#readme

https://github.com/NethermindEth/nethermind#readme

https://github.com/status-im/nimbus-eth1#readme

And there are many Open Source specs and libraries underlying those clients that have also involved non-trivial development and collaboration, e.g. libp2p.