This may be an obvious question - but since yunohost is an operating system it replaces Windows yes? And I see in the instructions that after a certain point the hard drive gets rewritten? I just want to confirm that means that if I’ve already moved some video files over (our wedding video that was going to get deleted from another location) that I should move them again temporarily before installing? Or am I not envisioning what is going to happen correctly? Thank you!
Yes, Yunohost is a complete Debian Linux based operating system and will overwrite whatever is on the disk. It can be installed in a virtual machine if you want to keep whatever operating system you currently have. Virtualbox is a free virtual machine app which runs on Windows so maybe try it like that if you have enough resources on your host.
edit: I use Windows only on my employer supplied laptop so not sure what it takes to install Nextcloud on Windows. But that might be more suitable for you if you want to keep your OS and current set of apps.