But what about if I use electrum wallet offline to generate seed phrase and address, is that not also called papar wallet?
Imho it's even better than a paper wallet, since at the moment of spending you will have change address too, making things easier, especially if you want to spend less than the entire wallet.
Also it's a list of English words, which are easier to write down to paper.
But there's a warning too: Electrum seed only works in Electrum and it's good to know that. Also if you want to be more widely compatible, using offline IanColeman page could be another similar direction.
If I created a wallet using the source from wallet like bitaddress or I use electrum offline to generate seed just like I said above, but I did not save the private key on paper but on flash drive, is that not called flash drive wallet?
The problem is that a flash drive is not a good and reliable option. One bad moment and it gets broken and you lose all the information. Also I don't know how many years it keeps that data. Multiple papers in geographically different places are cheaper and more reliable.