Documentation on the concepts are here:
Projects exist above Profiles (i.e a profile belongs to a project).
Almost all lxc commands accept a --project flag to allow specifying which project you want to operate within. Or you can use lxc project switch <project> to persistently switch your lxc command into a project.
You can use lxc project list to see projects and to see which one you’re currently switched into.
To edit a profile within a project you use lxc profile edit <profile> --project <project>