When dragging a file to another folder with the mouse, the file is copied instead of moved

A normal drag-and-drop between two regular folders on the same local volume should move the file. One thing to check first is the Ctrl key: holding Ctrl while dragging explicitly changes the action to Copy.
Then test with a small temporary file between two new folders, for example C:\DragTest1 and C:\DragTest2. You can also hold Shift while dropping the file, which forces a move operation, or use Cut and Paste as a reliable workaround. Microsoft documents that Ctrl+drag copies and Shift+drag moves.
If the test works but the original folders still copy, check the actual locations shown in File Explorer’s address bar. A shortcut, a redirected folder, a ZIP file, a network location, or a cloud-synced location can look like an ordinary folder even when Explorer treats it differently.
If even the test between two local C: folders copies, restart File Explorer or restart Windows and test again. Avoid registry “drag-and-drop fixes”; identifying whether Ctrl is being detected or whether one location is not a normal local folder is the safer first step.