Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
You should be able to access "C:" directly in File Explorer. One way would be to open the Run dialog box (hold the Windows logo key as you press R), type in *C:* and press Enter.
From within Word, use File > Open > Browse to browse for files.
That said, I suspect that the underlying issue here is that synchronizing with OneDrive is the default in most PC installations with Office. You can tell because the standard Documents folder is actually inside the local OneDrive folder.
You can move your files out of that synchronized location and put them elsewhere.
To set the local drive as the default for future Office files, start Word and go to File > Options > Save. Select the "Save to Computer by default" option and specify a "Default local file location."
Any existing documents that are already in OneDrive must be downloaded manually.
It is possible to turn off OneDrive synchronization for your computer as well. The unlink option is usually the best choice. See https://support.microsoft.com/en-us/office/turn-off-disable-or-uninstall-onedrive-f32a17ce-3336-40fe-9c38-6efb09f944b0 which provides instructions.