Is there an explainer somewhere for how releases work in the compose-multiplatform repo? I get frequent notifications of new releases, but almost none of them have any release notes associated with them

The releases are published following this scheme:

Compose 1.2.0 + changelog
Compose 1.3.0-dev547 (weekly or manually)
...
Compose 1.3.0-dev678
Compose 1.3.0-rc01
...
Compose 1.3.0-rc03
Compose 1.3.0 + changelog

right. Is there a guideline for what goes into those releases?

You can look at the list of the commits. And there is also a link to the list of the commits of submodule (the compose itself). It is complicated, so we want to atomate changelogs someday.