What the pattern actually holds
A QR code is a way of writing text so a camera can read it. For a website, that text is either the page address itself or a short link that points at the page address.
When the page address is the text, the code is static. Any scanner that reads the pattern gets the same address, and no other server has to stay online for that hop.
When a short link is the text, the code is dynamic. The scanner opens the short link, and that website decides where to send the person. The owner of the short link can change the destination, count the scan, or stop answering.
Why a free code can expire
A generator that offers a free dynamic code is usually selling the redirect. The pattern looks finished, but it only works while their short link works. A three-day limit is a limit on the redirect, not a limit built into QR codes.
Still makes both. A fixed code stores the address, and a scan does not call Still. An editable code stores a short link on Still, so you can change the destination later. It does not expire after a few days. It keeps working while this site is online.
When a redirect is the useful choice
A redirect is useful when the printed code must later point somewhere else, or when scan counts are required. Those jobs need a host you control, because the host can disable the code.
A menu, a poster, a product label, or a website button usually wants the opposite: the address you chose, still there next year. Use a static code for that.