Why will yarn install dev dependencies when I just need the builds?
Use --production=true (or simply --production or --prod for short). It is indeed normal behaviour; Yarn assumes you are in a 'development' context unless your NODE_ENV environment variable is set to 'production'.
Have a look at Yarn's documentation.
As said in the comment by @ddotsenko
Not "broken" but "badly designed" --prod still downloads and "installs" dev packages IF yarn needs to resolve "full tree". Just use
yarn install --production --frozen-lockfileand matching yarn.lock and --production will work as expected.
That worked to remove a 210 MB node_modules to 70 MB, similar to npm and pnpm.
Yarn has a --production option, which will cause it to install only production dependencies. This is shown here