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Apps can only be installed using Ledger Live. which must be online to download the apps from the server.
You can use some other front-ends that connect to the ledger to access your addresses, or even you could run tools like the ian coleman bip39 tool on airgapped amnesiac environment, to see all your addresses that can be derived from your seed phrase. But of course you won’t see the balances if you are offline, and such tools is risky if you are not fully sure of what you are doing.
The best is to use ledger Live to get your account addresses (or xpub in the case of UTXO like BTC), and once those are written down, you can view your balances using block chain explorer, with no need to use your ledger device.