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Yarn 1 (now called "Classic") used .yarnrc — same INI-style format as .npmrc, but read by Yarn instead of npm. It sets the registry URL, install behaviour, mirror caches, and other Yarn-specific options. The classic Yarn workflow has been mostly drop-in compatible with npm: same package.json, same node_modules layout, just a different CLI (`yarn install` instead of `npm install`).
Yarn Berry (Yarn 2+, released 2020) replaced .yarnrc with .yarnrc.yml. The new YAML format supports Yarn's plug-and-play architecture (no node_modules), zero-installs (commit dependencies to the repo as compressed bundles), and constraints (lint-style rules across a monorepo). If a project has both .yarnrc and .yarnrc.yml, or only .yarnrc.yml, it's using Berry.
Most projects on Yarn Classic could equivalently switch to npm, pnpm, or Yarn Berry — the package.json is the same. The .yarnrc itself rarely needs more than `registry "https://npm.your-company.com/"` for private registries. CLI is `yarn install`, `yarn add`, `yarn upgrade`. Yarn Classic entered maintenance mode in 2022 with no further feature development planned.