At the time of writing, namely v1.8.3, Biome does not fully support monorepos yet, but you can wrestle it into working.
Below, we have a typical monorepo (with the changes we're about to make marked in green). It has some workspaces under apps and packages, and at each level of the repo, there are a mixture of files we want to format or ignore.
.
├── .gitignore
+ ├── .vscode