The first one is a promotion method of merchants. Ordinary, not a liar. It is also common to send too many drinks in the street. Don't sweep around, you won't lose anything if you don't take advantage.
The second kind, like adding netizens, is mostly for making friends. You can add it in this case.
There is also a liar, who may cheat you after adding your friends. This kind of behavior is best not to be added, especially when many privacy issues are involved. As the saying goes, it is better to be careful than to harm others.
First, scanning a friend's QR code is not dangerous. Whether it is dangerous or not depends on the subsequent communication process. The biggest danger is that the other party lies to you and you fall for it. If you scan a fake QR code, it is very dangerous.
Don't trust strangers easily, learn to protect yourself.