boot – Where exactly is the bootloader installed?

I have two disks, the first with an NVMe controller and Windows 10. The second disk uses a SCSI controller, and I want to install Ubuntu on it. The disk and partition table recognized by the Ubuntu installer is shown in the figure below.

Partition table of the first disk (/dev/nvme0n1):

Partition table of the second disk (/dev/sda):

EFI partition, both in the disk partition is /dev/nvme0n1p1, /dev/sda3. The location of device for boot loader installation is /dev/sda3(efi). When I went into Ubuntu, I found /boot/efi is in /dev/nvme0n1p1 (efi partition of the first disk). And there’s nothing in /dev/sda3.

/boot/efi :

I wonder what causes this. When installing a dual system, the boot bootloader should be installed on a disk or an efi partition on the disk, and what is the difference between the two?