On a bitcoin exchange like Coinbase. Plenty of people have millions of dollars of Bitcoin. 10 million USD is about 2500 bitcoins, that'd barely move the market if purchased all at once, not at all if spread out over a day or two. 24 hour trading volume is about 3 billion USD.

doesn't coinbase require a link to a bank account? so you'd need to have millions in your bank account to buy the BTC.

The topic is moving capital across borders, that presumes you already have said capital and naturally yes that would be in a bank. Buying millions in bitcoin is not illegal, it will not be difficult, it doesn't matter if your bank reports the money transfers.