The end of NPSF3000's answer is basically the same questions I'd be asking. You say "only millionaires are allowed to live here"... so what happens if a citizen becomes a non-millionaire? Do they get deported, or killed, or otherwise very-bad-consequences? If so, then I wonder how different this would be from the real world, in which (I vaguely, viscerally, believe; and I don't think I'm alone in believing) there are bad consequences for letting your net worth reach \$0? In other words, is this world isomorphic to our world except that every citizen at birth stuffs \$1 million cash into their mattress and then lives the same life they would have had otherwise?
However, in the real world, there really aren't terribly terrible consequences for hitting \$0 net worth. For one thing, you can always get a loan, if your credit is good. If I have only $900,000 in my mattress, but I also have a \$200,000 bank loan (or the capability to get one), will your country still count me as a millionaire?
How do you even measure net worth? Right now my (real-world) bank reports that my net worth is negative, because I have more large loans outstanding than (they believe) I currently have assets-on-hand. But I also have a job with income, so my credit is good. I also have a certain amount of equity in a house, which the bank doesn't care to track as part of my "net worth": would your country count a house or a job as part of the net worth that goes into "being a millionaire"? Would your government outsource the tracking of net worth to some kind of credit bureau, or would the government try to track net worth itself, or what?
Actually, you know who frequently has a net worth in the deep negative numbers? Children. Would you need a rule that nobody's allowed to have kids until they and their spouse have a combined net worth of \$3 million, so that they could put $1 million into the kid's trust fund at birth?
What happens if a couple with net worth \$3.5 million accidentally end up with twins?
Does this country have a functioning government? (It must, right? or who'd enforce this specific rule?) If so, then the bookkeeping would become a lot simpler if the government just decreed that every citizen would henceforth "own" \$1 million from the government's own vaults — sort of like "40 acres and a mule" crossed with the elevator pitch for Tether. Then no citizen would ever be in danger of becoming not-a-millionaire, and everyone could just go about their business as usual.
In short, this whole idea seems like it needs a lot of details fleshed out; it's very complicated to implement. And because it's so complicated to implement, it raises the important question of why — why does your country work this way? What's the benefit? Why would anyone go to all this trouble?
Figure out the why (in-universe), and you might get a lot further along in answering the rest of the how.