1) Which files? LibreELEC's system files are on the readonly system partition in /flash.
2) LibreELEC has its own backup/restore tool in the Settings Add-on. It includes the thumbnail cache folder. However, it does not backup the entire LibreELEC system section. You can use proper disk imaging tools for that. Transferring/copying Kodi setups to other computer OS clients is not 100% possible. Windows and Linux folder paths really do not get along. Also binary add-ons do not mix on a different OS.
3) Please note that the word "build" has pretty negative aura on it when it comes to Kodi. More often than not do such "builds" have video piracy add-ons onboard, which is a big red flag for Kodi and LibreELEC teams to halt their support. I hope this is not the case with your Kodi setup. Such add-ons are known to disrupt Kodi core functionality, who knows even the connectivity to your Kodi remote app.