• When you create a software key it gives you the seed like any normal wallet. 
• If you are using a Coldcard then it’s just the Coldcard seed, there isn’t a new one for the wallet.
• If it’s multisig you need to also save a copy of the wallet config. I always simply email that to myself, it’s needed to restore, but isn’t terribly sensitive, I prioritize availability over privacy there. Although I send from a protonmail account to another protonmail account with the subject “BACKUP” so it’s just always searchable in email if I need it.

There is nothing uniquely different about Nunchuk and key backups. It forces you to save your seed during the key creation process like any of them. You might be remembering incorrectly or confusing how you set it up?