I don't have much inside info, but the contents of the QR codes on Alibaba, JD.com, and others differ substantially, so I suspect each is building their systems in-house, though in China, I would think there's more of an established playbook for building them given its prevalence. I haven't run across a dedicated library/package for this.

In the West, WhatsApp and Discord have massively different schemes for QR login.