Generate a QR code without uploading what it encodes

The payload is the whole story. People encode a wifi network name together with its password so guests can join, a contact card with a home address and a phone number, a link to a document that is not meant to be public. All of that is the content of the code, so a hosted generator receives it. Guests scanning your wifi is the point; a generator holding your wifi password is not.

There is a second difference that only shows up later. Many hosted generators produce a code that points at their own address and forwards to yours, which is how they count scans and how they retain the ability to change the destination afterwards. That also means the code stops working if that service does, and it prints somebody else's address onto your poster or your packaging. A code generated locally encodes your destination directly, with nothing in between.

The trade is that a static code cannot be changed once printed. On a business card or a piece of packaging that is a feature, because nobody can repoint it. On a poster you might want to update, it is a limitation, and the answer is to point the code at an address you control and change what sits behind that address.

Verify with the network off, which is easy to do and slightly satisfying: the code renders anyway. Then test the printed result with more than one phone in ordinary light, because ink, paper, size and contrast all affect scanning in ways the screen version never shows you.