I just started using Flutter and I am trying to create a crypto wallet app that can support multiple different cryptocurrencies. The current issue I am facing is, is that I don't know how a mnemonics code is being generated for different cryptocurrencies when trying to create a backup for multiple wallets. I am using this library btw: https://pub.dartlang.org/packages/bip39
Any help would be much appreciated!
You can use one mnemonic code for all cryptocurrencies. As you can see in the library documentation you linked, you use the mnemonic code to get a wallet seed. That wallet seed is then used to create a HD wallet.