There is a config parameter to rescan you files on each request.
I used it once after i copied files into the datadirectory via cli an nextcloud did not recognize them.
You should only enable it temporary, since its is slow but it might help in your case if you do it once.
https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/config_sample_php_parameters.html#filesystem-check-changes

You need a software like rufus or etcher for windows. Run the software as admin, select your drive (be careful to select the right one. It will be wiped in the process), select you iso file of linux mint and confirm. The software will create a bootable usb drive. Plug it into you pc and reboot. Now the process ist depending on your computer. Some will boot right into the live usb others will boot back into your normal hard drive. If later one is the case, you need to enter your boot menu via one of the function keys. Which one depends on your mainboard.

If you still have questions, feel free to ask. And have fun with Linux :D