The short answer
Steam is the modding platform: full file access, the official (experimental) Workshop and every tool working out of the box. Game Pass PC (WinGDK) runs most mods but everything is manual. Xbox consoles cannot be modded at all — there is no file access, no loader, no pak folder.
Side by side
Steam: binaries in Pal\Binaries\Win64, .pak mods, UE4SS + PalSchema (Okaetsu build), Steam Workshop support, and mod managers like the CurseForge App that detect the install automatically. Game Pass PC: binaries in Pal\Binaries\WinGDK, .pak mods and UE4SS work if the mod author supports Game Pass, but no Workshop and no manager detection — everything goes in by hand. Xbox console: nothing. If a guide doesn't name a platform, it means Steam.
Quirks that bite Game Pass players
The Xbox app can revert modded files during updates or repairs, so keep your mod zips outside the install folder. Some 1.0 features skip the WinGDK build entirely (dedicated-server voice chat is the documented example), and script mods built only for Win64 may crash or silently do nothing on WinGDK — check the mod page for Game Pass support before installing.
Playing together across platforms
Cross-play sessions follow the server's rules: a modded dedicated server expects clients to mirror its mod list, no matter the store. Console players joining a modded server are the exception — they can only join servers running mods the game allows without client files; when in doubt, assume unmodded servers for console friends.