New services and apps should be added in their respective package paths, packages/backend/ and packages/frontend.
To compile with Typescript, they'll need their path added to the root tsconfig.json, e.g. references: [{ "path": "packages/backend/new-service" }].
Due to Yarn workspace config, dev dependencies are shared between projects and hoisted to the workspace root. If this presents problems, a nohoist config for specific packages can be used - see more.