What needs to happen before the Beacon chain merges with Mainnet? Is it Vitalik pushing a button? A certain contract that needs to reach a certain amount of staked Eth? Some amount of burned Eth? Or is the beacon chain not ready yet and needs to be tested and/or developed?
You can see on the Ethereum's repo on github a to-do list of all the things done and what remains do to be done before the Merge.
An example of what still needs to be done is to make sure that during the merge, every pair of (Conensus client, Execution client) works seamlessly. For this purpose the Ethereum Foundation makes shadow forks. You can see on the mainnet-shadow-fork-6 what are the results of the last shadow fork.