Question Windows 10 installation didn't delete previous files

[Alexxx99]

Ok,so friend of mine gaved me his pc to reinstall his windows 10.When I first time installed it there was an error while installing some programs.Our friend who was pc serviceman told us that the problem was that his new username had some letters which weren't in english alphabet and that made that weird error while installing some programs on new system.I booted windows 10 installation from usb and installed windows 10 again on his computer.When I clicked on c partiiton to install there was a warning that everything on that partition will be deleted.
That weird bug was fixed but some bug is happening again while installing very specific program and my friend told me that some files like songs and videos has stayed on his system.
Pc serviceman said that I didn't format disk and that's the reason why it is retained some bugs from previous installation.
How is that possible when I install it there was a prompt that everything on that partition will be deleted,why do I need to format?

 

[USAFRet]

Whatever you did, you did not delete all existing partitions.

Start over, exactly like this:

How To - Windows 10 clean install tutorial

If you are looking for the Windows 11 Clean install tutorial, you can find that here: Windows 11 Clean install tutorial (Click here) Otherwise, welcome to the Windows 10 Clean install tutorial This tutorial is intended to help you, step by step, to perform a clean install of Windows...

forums.tomshardware.com

 

[Alexxx99]

He has two partitions,C where windows is installed and D for movies and pictures.Can I,during installation just delete C partition and System reserved partition or do I need to format them and will deleting system reserved partition affect usage of D partition?

 

[CountMike]

He has two partitions,C where windows is installed and D for movies and pictures.Can I,during installation just delete C partition and System reserved partition or do I need to format them and will deleting system reserved partition affect usage of D partition?

Yes you can delete ačč partitions but D: which will be preserved.

 

[USAFRet]

He has two partitions,C where windows is installed and D for movies and pictures.Can I,during installation just delete C partition and System reserved partition or do I need to format them and will deleting system reserved partition affect usage of D partition?

Is that data backed up anywhere?
If not, why not?