What does the error-correction level change?
It trades code density for damage tolerance. A QR code with higher error correction can still scan if part of it is scratched, faded, or covered by a logo, but it packs the same data into more modules, so the printed code gets denser at a given size. M (15% recovery) is a sensible default; go higher only if the code will be printed small, laminated, or handled roughly.
Do QR codes made with a free generator expire?
No, a QR code generated here never expires or stops working. The code encodes your text or URL directly; there’s no third-party redirect service involved that could later go offline. That’s a different situation from some design-tool QR codes, which is covered in whether Canva’s QR codes expire.
Which format should the code point to?
For a URL, use the full address including https://. Most scanner apps only offer to open a link if the text is unambiguously one. For anything else, like a phone number, a plain message, or a Wi-Fi network, plain text works, but check the reading app supports it before printing something you can’t easily reprint.