Steam Achievements Don't Sync to Xbox 360 (Portal Example)

No. Steam (PC) and Xbox 360 (Xbox Live) are completely separate ecosystems. Achievements do not cross-platform.

Why

• Different services: Steamworks vs. Xbox Live. No shared identity, no API bridge.
• Different lists: Portal on Steam has ~30 achievements. Portal: Still Alive (XBLA) has 12 different ones. Portal 2 (Steam/PS3/Xbox 360) has platform-specific lists.
• No account linking: Valve and Microsoft don't merge achievement data.

What You Can Do

• Earn Steam achievements: Enjoy Portal on PC. They're tied to your Steam profile forever.
• Buy the Xbox version: Portal: Still Alive (XBLA, delisted — need code/used account) or The Orange Box (disc, has Portal with 360 achievements).
• Xbox One/Series: Portal not BC. Portal 2 is BC (via The Orange Box disc or digital if previously owned).

Cross-Platform Progression Exists For:

• Fortnite, Rocket League, Destiny 2, CoD: Warzone, Minecraft (Bedrock), Apex Legends — via Epic/Activision/EA/Microsoft accounts.
• Xbox Play Anywhere titles (buy once, play PC/Xbox, shared saves/achievements). Portal is not one.

Enjoy Portal on Steam. It's the definitive version (workshop mods, HD textures, commentary).