Default [Q] Unlock codes, warranty, binary counter, knox, OTA updates

Greetings, I told my carrier that I wanted to use a unlock code to use my phone in a trip overseas with a local carrier.

My carrier told me that unlocking the device would void the warranty.

If I have to send my phone to service, would they be able to tell that I have used an unlock code(besides putting another carrier's sim)?

Does using an unlock code modifies the binary counter or the knox flag?

Does using an unlock code takes away the official status of my phone? Will I still get OTA releases?

What sites do you recommend to get an unlock code?

Thanks so much in advance.