This is a monorepo where there are several subprojects in py/release-*/projects/ and a top-level virtual project (for each pants resolve) that has a pyproject.toml and poetry.lock file.
Each workspace gets it's own dependency group so if you want to install a single workspace (e.g. to run unit tests in CI) you can run poetry install --only cli which will install cli and lib (because cli depends on it).
If you run poetry install you get all groups which is probably what you want for local development.
This monorepo also supports sandbox projects which are ignored by pants by can install libs using path dependencies