Is it even possible to create a hardware wallet and integrate it to a normal USB pendrive.
Simple answer, no.
I am talking about copying the code embedded in hardware wallets like Trezor and Ledger Nano S and then integrating it to USB drives.
That's not possible.
A hardware wallet consists of more than just a flash memory chip.
1. The above devices have a built-in screen that protects their device with a password. Now obviously our USB drive won't have that feature. If we remove that feature and implement the other features would that be doable thing ?
No, also you need some kind of verification which happens outside of your computer you are using to transact (preferably only on the hardware wallet itself).
2. Basically, the hardware devices are just USB drives with embedded code. Why can't it be copied and integrated on other USB drives ?
They are not. Hardware wallets have secure elements embedded which keep the secret information secure and do the signing operations.
Like your credit card. It doesn't only consist of some plastic with a small flash memory.
3. If it is possible why haven't we seen people doing it ?
Because it is not possible.
We have seen people who think it is possible, which simply just copied their wallet file onto the usb. But those people don't have a clue at all.
4. Can't a hacker get hands on one of these device and manipulate the code to get access to the wallet ?
Well I do know that it is encrypted through cryptographic algorithm which makes it almost impossible to hack the wallet by manipulating the code but I would like more info on this.
I guess you are talking about hardware wallets now (not usb sticks) ?
IF some attacker is able to flash the firmware of a hardware wallet without erasing its secret information (seed), yes.. definitely possible.
But you can't simply flash the firmware to get your own version on it without erasing all relevant data. If you would be able to do so, that would be quite a severe vulnerability (which already existed in ledgers hardware wallets, but got fixed).
If you are not able to flash the firmware, you have to somehow get the secret information out of the device with the official firmware installed.
And this is not possible, given that there are no vulnerabilities (which could always exist; especially side-channel attacks etc.. ).