There are several sites that allows you to create a wallet for bitcoin offline- for extra security
How is this possible? If you are offline then you are not interacting with bitcoin blockchain ...
Does anyone knows how this work ?
You don't need to interact with the blockchain as the generation of keys and addresses are a specific algorithm (SHA256) using
elliptic curve mathemathics to generate the public key (K value) and address (A value). Bitcoin has it's own generic point on this curve to calculate K and A values, as does Ethereum and any crypto coins, while the "small k" value is a randomized point on this curve, for obvious security. To summarize: you don't need the internet (or even a computer!) to calculate mathematics that the Bitcoin blockchain recognizes, as long as you know the point on the elliptic curve to calculate K and A values; public key and address.
Proof of Bitcoin ownership is from the private key only (a type of password), which can therefore be saved on a desktop or online, or simply stored offline.
If you're new, this probably makes no sense, and is one of the most difficult parts to comprehend in my opinion. It took me a long time personally!