Create boot EFI files

If your system is configured to boot Windows in UEFI style, then there should be a partition on a GPT-partitioned disk, formatted as FAT32, and with a partition type GUID that marks it as an EFI System Partition, or ESP for short. It should be typically sized between 100 and 260 MB. The partition is normally hidden and does not have a drive letter, so its existence may go unnoticed unless you specifically go looking for it in Disk Management.

If you boot from a Windows 10 installation media, there is a "Repair your computer" option on the same screen as the "Install now" button that begins a regular installation process. It should detect the presence of your Windows system partition and attempt to reconstruct the UEFI bootloader.

But be careful: in order to successfully repair a UEFI-style system, the installer must be booted UEFI-style too. If the repair process fails, you might have to use some bootable partitioning tools to create an empty partition of suitable size and mark it as an ESP partition first, then re-run the repair process.

Most of the Windows bootloader files on the ESP partition are generic and could simply be copied from another installation. But there is one file named BCD which contains information specific to your installation, and it must be reconstructed if lost. The repair process can usually do that automatically.

Windows ties the choice of boot style to the choice of partition table type: if your system disk uses GPT partitioning, then you must boot UEFI-style; if your system disk uses legacy MBR partitioning, then you must boot legacy MBR style.