I think you’ve got a bit of confusion in what needs alteration.
Apple doesn’t offer much clarity here on what the structures are. Both HFS+ and APFS files systems use GUID which is the partition type.
The Windows partition is an independent partition so you would first need to wipe the partition out from within Disk Utility so the space is empty and then extend the macOS partition to include it. Now the fun part! APFS is not complete! Apple has not finished developing it or offering the needed tools to work with it !!
Apple figures your task is rare as such has not offered any way to do it yet. They figure you’ll just do a TimeMachine backup, reformat the drive and then restore.