Mac storage basics · Checking your storage
Why are there two volumes, Macintosh HD and Macintosh HD – Data?
Short answer: Macintosh HD is the sealed, read-only macOS system; Macintosh HD – Data holds your apps and files. They share the same space, and the Data volume must never be deleted.
Since macOS Catalina, your startup disk is split into two volumes in the same APFS container: Macintosh HD holds macOS itself and is read-only and sealed, and Macintosh HD – Data holds your apps, files and settings. Finder shows them as one drive, which is why you normally only see Macintosh HD.
They share the same free space, so there's nothing to resize and nothing to delete. Never erase or delete the Data volume — that is where everything you own lives.