Technically what will happens to make the merge ?

Hi all,

Been searching a while for the coming merge, I still can't find anywhere saying about what are technical steps for the merge to happen. Let me summary current situation:
- Mainnet: PoW, at block 15,4xx,xxx, hundreds of miners (pool), multiple version of ETH clients
- Beacon chain: PoS, at block 4,5xx,xxx, 417,xxx validators/producers, centralized ETH client with different versions (guessing)?

I can imagine, in order for the "merge block" in Beacon chain to take information (header) of the last block on Mainnet and include that information on Beacon chain:
- The merge block number to be defined in last version of ETH client in Beacon chain
- Most of validators/producers should download and run that same ETH client version in Beacon
<= if this is the case, how the merge database will be ? will it be a combined of both databases or only on top of Mainnet database (exiting Beacon database will be drop) ?

This simple mindset is coming from old understand of BTC. I think it is too simple now. So please explain me the real situation of the ETH merge.

Thank you!