This multiplayer roulette game uses a provably fair method, which means that the numbers/spins cannot be manipulated by anyone, nor PlayNano nor any of the players.

So how it works?

When a round starts, PlayNano server's automatically generates a small random string which we'll call secret. This secret will only be revealed once the number/spin is calculated, hence the name, secret...
The server will hash the secret using the SHA256 hashing algorithm. This creates an hash, let's call it secret_sha256, that allows you to verify the secret once it's revealed. It's impossible to determine the secret by having this secret_sha256, but has soon as the secret is revealed, everyone can confirm that the secret wasn't manipulated and did generate that secret_sha256.

So, we've covered the server's secret data, now we need some public data, easily verifiable by all users. For this you'll need to understand a bit further how this roulette works.

Every round has a unique main Nano address. You never send Nano directly to this address. When you submit a bet, the server also generates a unique address for you. If you send the correct Nano amount to that address, your bet will be accepted, and, you are creating a send block on the Nano network, and that block has an hash... see where we're going? We'll use all hashes from all accepted bet blocks!

So, how one verifies this? When a round is over, the main round address will start receiving Nano from all those accepted bets' addresses, including yours. From that main address, you can go through all the bet addresses, and then all the hashes of the send blocks. (I know, this is boring, but seems to be the best method to prevent any manipulation, so...)

Now that we have all this data, we just hash everything, secret and all those public block hashes (SHA256, here we go again!), pick the first 8 digits, and calculate the number from that. Those 8 digits are hexadecimal, so we divide them by 0x100000000, to get a number between 0 (inclusive) and 1 (exclusive) and multiply by 37. Then we'll ignore the decimal cases and baamm!, we have a random number, between 0 and 36, that no one could manipulate!

The number generated cannot be manipulated by any user, even if playing alone, because of the secret, and the number cannot be manipulated by the server because of those public, user generated, block hashes (which include yours, if you did bet that round).

I promise I'll improve this explanation in the future! ¯\_(ツ)_/¯
If you didn't understand, try contacting me and I'll try to explain it better.