The easiest official method does not directly give you a .dmg file but an .app (application) bundle. You can then extract the DMG from this app.
Big Sur officially supports:
There are three main official ways to secure a legitimate copy of the Big Sur installer without resorting to risky third-party mirrors. Macos Big Sur Dmg Download
directly from Apple's servers using links often curated by reputable community databases like Mr. Macintosh The easiest official method does not directly give you a
