In one of the most common use cases with a UEFI Windows PC you have a nominally-hidden ESP (boot) partition or other well-recognized FAT volume containing a /EFI/boot folder with its /EFI/boot/bootx64.efi file which is a UEFI executable.
When you install Windows or run the BCDBOOT command correctly it (over)writes the /EFI/boot/bootx64.efi so it then points to the accompanying /EFI/Microsoft folder which then runs the Windows BCD (multi)bootloader which can boot to any Windows installation you have on any Windows partitions. The built-in Windows multi-boot menu only appears when you have more than one Windows installation on different partitions. Haven't found a way to boot Linux from the Windows UEFI bootmenu though, it was easily done in BIOS, but not native UEFI.
When you install something like Ubuntu it (over)writes the common /EFI/boot/bootx64.efi file so it then points to a new accompanying /EFI/ubuntu folder containing a /EFI/ubuntu/grubx64.efi file.
Grubx64.efi then accesses the bootmenu contained on the Linux partition it was associated with during the most recent grub installation and/or grub update (as specified in the /EFI/ubuntu/grub.cfg text file).
The grub bootmenu usually is displayed since there are usually more than one alternative boot option with the default being the normal Ubuntu OS boot. Also normally the Ubuntu install will automatically include a reliable grub boot entry for the Windows system which is already in place.
So the Linux UEFI bootloader will ideally load either Windows or Linux from a Linux bootmenu upon startup. Even with Microsoft SecureBoot enabled.
But the Windows bootloader will no longer do either type OS without reverting to MBR-style partitioning and traditional BIOS or CSM booting.
When you install Windows or run the BCDBOOT command correctly it (over)writes the /EFI/boot/bootx64.efi so it then points to the accompanying /EFI/Microsoft folder which then runs the Windows BCD (multi)bootloader which can boot to any Windows installation you have on any Windows partitions. The built-in Windows multi-boot menu only appears when you have more than one Windows installation on different partitions. Haven't found a way to boot Linux from the Windows UEFI bootmenu though, it was easily done in BIOS, but not native UEFI.
When you install something like Ubuntu it (over)writes the common /EFI/boot/bootx64.efi file so it then points to a new accompanying /EFI/ubuntu folder containing a /EFI/ubuntu/grubx64.efi file.
Grubx64.efi then accesses the bootmenu contained on the Linux partition it was associated with during the most recent grub installation and/or grub update (as specified in the /EFI/ubuntu/grub.cfg text file).
The grub bootmenu usually is displayed since there are usually more than one alternative boot option with the default being the normal Ubuntu OS boot. Also normally the Ubuntu install will automatically include a reliable grub boot entry for the Windows system which is already in place.
So the Linux UEFI bootloader will ideally load either Windows or Linux from a Linux bootmenu upon startup. Even with Microsoft SecureBoot enabled.
But the Windows bootloader will no longer do either type OS without reverting to MBR-style partitioning and traditional BIOS or CSM booting.