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.

Instead of dragging, right-click the file, select Cut, then navigate to the destination folder, right-click, and choose Paste.

You're dragging files between folders you believe are on the same drive, but File Explorer copies instead of moving them. First verify both locations' full paths and volume letters; libraries, Quick Access, OneDrive, mapped folders, and junctions can look local while targeting another volume, where copy is the default. During the drag, watch whether the tooltip says “Move to” or “Copy to.” Ensure Ctrl is not being held or stuck, because modifier keys can change the operation. Try right-button dragging; Windows will display a menu where you can explicitly choose Move here. For a dependable workaround, select the files, use Cut, open the destination, and Paste. If ordinary same-volume folders still copy, test another mouse and keyboard, restart File Explorer, and repeat with two new local folders. Avoid changing registry drag-and-drop values until both paths are confirmed. Share the paths if the behavior continues.