If you install a fresh install of Windows on a new hard drive. Yes, you will have to reinstall all your software.
As for your files. No, you don't have to place them on the new hard drive. You can have them on any hard drive you wish. All you need to do for your files is know where they are located. You can make any shortcuts you wish.
To access any files on your old boot drive. You will have to take ownership and fix permissions. As the new install will not recognize you as having authority to view them. As for accessing files from the backup drive. That all depends on how your backup program works.
https://winaero.com/blog/how-to-take-ownership-and-get-full-access-to-files-and-folders-in-windows-10/
If you are replacing your boot drive with an SSD. Many of them include cloning software (via download). In which case you just clone your boot drive. Then you don't have to mess with anything. After cloning the drive. You just shutdown and unplug the old drive and boot off the new SSD. Continuing where you left off.
Just to clarify. You do not need to buy a new copy of Windows. Your post indicates that may be your intention. You can reinstall the same version of Windows as many times as you want on the same computer under the same license. If it is Windows 10. It will activate automatically.
Download Windows from MS:
https://www.microsoft.com/en-us/software-download/home
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