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First a small correction:
> So I own Ledger Nano S and I currently have stored ETH on it
The ETH are not stored on the device. The device only holds the keys that let you access your funds.
> but I have a lot of other smaller crypto like TFUL, BTT, BABYDOGE and etc. How do I store them on my Ledger Nano S if there’s no app for them like with ETH and Stellar for example? Or should I go and buy Ledger Nano X?
If there’s no app for it, then it’s not supported. The full list of supported cryptocurrencies can be found here: https://www.ledger.com/supported-crypto-assets
The Nano S and Nano X support the same coins, but with the Nano X you can have more apps on the device at the same time.