This isn't unique to the Ledger Nano X, Ledger products in general, or even hardware wallets in general. Almost every wallet you can use these days, be it hardware, software, mobile, etc., uses seed phrases to both generate the wallet and to let you back it up. Pretty much the only time you won't use a seed phrase is if you use a custodial wallet such as a web wallet or exchange, in which someone else is holding your coins on your behalf. It is a set standard across the entire bitcoin ecosystem. You can read more about seed phrases here: https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki

Only ever store it on paper and never on an electronic device. Keep it secure. If you are worried about people discovering it, then there are a number of options available to you - split it up in to a m-of-n secret share, so an attacker needs to find multiple pieces of paper rather than just one, encrypt it before you store it (but make sure you also back up your decryption key), or (the most preferred method) use an additional passphrase in addition to the seed phrase (again, make sure you also back this up on paper separately to your seed phrase). There are instructions on using a passphrase with Ledger wallets here: https://support.ledger.com/hc/en-us/articles/115005214529-Advanced-passphrase-security

Seed phrases generated by Ledger use the BIP39 standard I linked to above. This is widely supported and can be restored to almost any wallet currently available.