QwerkyPengwen :
In short, whatever files are on the OS drive will be erased, that is, if you were to be using an older version of Windows 10. Stuff on a separate drive that doesn't have the OS on it will be left alone.
Follow this guide here to be able to do a reset that will affect the OS and installed programs and applications, but will leave files alone. Be sure to back up any important files though just in case.
https://www.laptopmag.com/articles/reset-windows-10-pc
I believe it keeps the files in specific folders on the OS drive like "Documents" folder... I can't confirm this but I saw this posted:
"Here's what is and is not saved:
Files in your user profile folder (C:\Users\YourName) are saved as expected. It doesn't have to be in a sub-folder (Documents, Pictures, etc) as files saved directly in the profile folder are saved as well.
Files in the system drive are saved. That is, files in C:\ (if C is your system drive) and sub-folders that you created there are safe.
Desipite being in your user profile, AppData is wiped (C:\Users\YourName\AppData). Files there are NOT saved.
Files in Program Files, Program Files (x86), ProgramData, and Windows are NOT saved. This is expected because Windows says "apps" are removed. Windows will make a list of programs removed and put it on the Desktop for you after the reset.
Files stored directly in the users folder (C:\Users) and not in a specific user profile are NOT saved.
The Public user profile (C:\Users\Public) is NOT saved."
Bit messy, but that should include:
Documents
Downloads
Pictures
etc (in the "username" folder)
Probably a "cloud" folder like OneDrive that would have files kept too (or at least that should have a copy on the server which it copies back again).
OTHER:
Reinstall the missing programs etc and if the PC is running great now you may want to:
a) run disk cleanup-> cleanup system files (right-click C-drive) then look for large stuff including the "OLD" Windows folder (used to rollback after some major updates/changes)
b) make a backup IMAGE with a tool like Acronis True Image of the C-drive (highly recommended)