How to see where the grub bootloader is?

Question

GRUB

Good time. Recently I looked through the bootloader in Secure Boot on my laptop - there are a lot of remnants from operating systems that were, but no longer are. I looked through gparted - there is nothing on the disk. But there is something somewhere, since it is displayed in the bootloader, even after the complete demolition of partitions, and it is clearly on the disk, since the command dd if=/dev/zero of=/dev/sda completely demolishes these remnants, and it works with disk. So how can you see and remove the old bootloaders on the disk?