I have 2 btc on Coinbase and send them to 2 separate addresses. How on earth can they know they are both mine without asking Coinbase? Seems fishy.

I do not have and never would ever have 1₿ in one address or as 1 UTXO. Rich or poor, it's just so unlikely you would. Especially a pleb. Especially post 2017 when buying whole coins is not an option for 99% of people. So... What they do is put together a made up methodology to estimate, then cha...