If you have generated a seed phrase or private key, the wallet you used can give you the option to backup the seed phrase or private key using QR code. You can use the QR code as backup. So that anytime you want to recover the wallet you can use the QR code backup to recover your wallet as it contains the seed phrase or the private key. But I will not advice you to use this method for backup. It is better you write down your seed phrase instead which is better and seem more secure in my opinion.
You can always restore a simply written seed phrase (on paper, metal or any other way) without third-party applications. In the case of QR-code backups, you will need some kind of application that converts from one format to another. This moment is the most unsafe. Why create an extra (dangerous) intermediary between you and cryptocurrencies?
I always go by the old school method of writing on paper but to answer the question QR code should convert the format in completely offline so if someone using an air-gapped device then it's same level of secured as writing on paper, but if the device can expose to internet while reading or even after we done we can't say for sure that our seed is completely safe with us.