Most "upd" listings point to a mix of legal high-res storefronts and community-driven archives:

The Evolution of K-Pop High-Fidelity Downloads (Updated 2026)

He leaned back, closed his eyes, and smiled. The "upd" was perfect. He’d be tired at his desk job in four hours, but for now, he had the best seat in the house. 💾 Why FLAC Matters for K-pop

That being said, here's a blog-style post with some general information: