NDK is structured as a monorepo using pnpm as the package manager.
git clone https://github.com/nostr-dev-kit/ndk
cd ndk
pnpm install
pnpm build
If you only care about building ndk core and not the family of packages you can just
git clone https://github.com/nostr-dev-kit/ndk
cd ndk
pnpm install
cd ndk
pnpm build