K13 Max

The wireless Keychron keyboards' source code is not part of the official QMK project (and probably never will be). It is in a separate fork (there is more than one fork, but only one is the primary). There are multiple Git branches within the main fork. For example, for a K Pro series keyboards, the

Re "K13 Max and QMK support - nonexistent?": No, it is just that Keychron dumps the source code on GitHub (very, very late) and don't tell anyone about where it is, what it contains, how QMK needs to be set up, when they update or release it, and what the main version is (and what part is stale).