The merge is at the point where real interop testing can begin with testnets spun up involving multiple clients. This video shows a testnet running locally with four nodes:

• Teku & Geth
• Teku & Besu
• Lighthouse & Geth
• Lighthouse & Besu

Each node has 1/4 of the validators and the network transitioned through from the initial phase0, to Altair and then then merge after which the two chains merge into one.  Also shows sending transactions with MetaMask post-merge and how the user experience is largely unchained. Ethereum keeps on being Ethereum but now with PoS instead of PoW.