Anyone with a private key (and the knowledge on how to use it) can spend any bitcoins that are associated with that private key.
I would like to know this for safety.. I know they can steal your btc by having your private key,
For safety, that is all you need to know. Protect your private key. If someone else has it, they can spend the bitcoins. If you lose it, you cannot spend the bitcoins.
but do they add the private key to their own then?
Ah, so this isn't just "for safety". You want to know what to do once you have the private key. Are you planning on trying to steal bitcoins from others?
Yes, importing the private key to their existing wallet would be one possible method of allowing you to create transactions that spend the bitcoins associated with that private key.
Or do they make a new btc wallet and replace the private key with the one on tv?
Sure, that would work too.
And how do you use that present card he gave on the last?
The last what?
How is it located to your own btc wallet by just scanning that with your QR reader?
It depends on the wallet you are using. Some wallets allow you to import a private key by scanning a QR-Code. Others require you to enter the text of the private key.
step by step guide plz
?
Step by step guide will depend on what you have and exactly what you are trying to accomplish. There are dozens (maybe hundreds) of ways to use a private key.
You can either take the time to learn the technical details of exactly how bitcoin works so you can figure them out for yourself, or you can describe exactly what you have and are trying to accomplish so someone can help you accomplish it. But there is no way I (or probably anyone else around here) is going to take the time to write a few dozen (hundred?) step by step guides on every possible way to use a private key just to humor you.
