You should first be aware that modern EFI-based computers boot in entirely different ways from older BIOS-based computers. I suspect you're laboring under incorrect BIOS-mode mental models, which is leading you astray. I recommend reading:
That's admittedly a long reading list. If you have time for just one, I'd go with the first, or possibly the second. The third and fourth are more practical tutorial on installation, but you've already got a working system, albeit one that's not working in quite the way you want.
Of particular importance for this discussion, under EFI, boot loaders are stored as ordinary files in the EFI System Partition (ESP). The boot loader to be launched by default is stored in NVRAM, which can be displayed by
sudo efibootmgr -v in Ubuntu, and modified with other options to
man efibootmgr for details, in the usual terse manpage style.) Your
/boot partition holds Linux kernels, GRUB configuration and support files, and a few other boot-related tools; but the main GRUB binary,
grubx64.efi, is stored on the ESP, which is mounted at
/boot/efi in Ubuntu.
That out of the way, Windows requires the Windows boot loader, and Ubuntu requires a Linux boot loader. The latter is GRUB 2 by default, but can be any of several other EFI boot loaders for Linux. The task of a boot loader is to load an OS kernel into memory and start it running.
A boot manager, OTOH, displays a menu or otherwise provides a way for a user to select what OS to launch. GRUB and the Windows boot loader both provide boot manager functions as well as boot loader functions; but there are programs that provide boot loader functions only or boot manager functions only. In fact, all EFIs provide some sort of boot manager, although in some cases it's so primitive that it's useless.
I've heard that it's possible to configure the Windows boot manager to launch GRUB, even in EFI mode; but I don't know the details of how to do this. You might ask about it on a Windows forum if you want to investigate this approach.
Another alternative, if you don't like the GRUB UI, is to look into another boot manager. For something that's non-intimidating once configured, my own rEFInd boot manager may be to your liking. It presents a graphical view (although it's still keyboard-driven), so it's likely to be less intimidating than the text-mode GRUB. You might want to tweak rEFInd's configuration once it's installed. See its configuration and Secure Boot documentation in particular.
Find UEFI boot entries
To remove the now useless Windows boot entry in the UEFI, first you need to find it. This can be done directly from the main Ubuntu OS. There is no need to boot from a live USB/DVD. The installed Ubuntu already has the required commandline utility.
Open a terminal by pressing Ctrl+Alt+T and then enter:
Enter the password when prompted and hit Enter. The cursor won't move and you won't see any asterisks(*****). This is normal in Ubuntu terminal. You will see some output like:
Note, I got two entries other than windows,
0for "Dell" and 1 for "Windows Boot Manager". Since I deleted all the original partitions except EFI, I can no longer boot "Dell" or "Windows Boot Manager." I could delete both
1. But for this answer I will show how to delete only the Windows Boot Manager entry.
Note: the entries you want to delete may be different than this example. Use the output you get from your own computer, not the above example, to decide which entries you want to delete.
Delete the obsolete boot entries
Now that you know you want to delete the entry
1(in this example), In the terminal enter:
1in the command above. This command will delete the entry:
Delete the corresponding folder from the EFI partition
We are almost done. This last step makes sure the
UEFIdoes not regenerate the entries we just deleted. To do this you have delete a folder corresponding the entry you deleted in the previous steps.
Now you find the name and location of the folder to delete. Use the following command to list all the folders in the
EFIsubfolder in the partition:
You will see some output like:
To delete the
Windowsfolder use the following command:
To verify that the folder was deleted, use the
sudo ls /boot/efi/EFIcommand again.
Finally update GRUB to hide the GRUB menu
At this stage when you boot the laptop, GRUB will still show the menu to choose various OSs, even though you have only Ubuntu installed. Edit the file
/etc/default/grubwith the following command. You may use any other editor if you want.
Make the two lines look like this:
If the line
GRUB_TIMEOUT_STYLE=hiddendoes not exist, add the line as above.
When done, save and exit the editor by Ctrl+X, followed by Y and Enter.
The final step is to update GRUB with the following command:
Now the laptop should directly boot Ubuntu without showing the GRUB menu. If you need to get to the recovery mode in the future, then hit Esc while the laptop boots to reveal the GRUB menu.
Reference: This answer is based on How do I remove "Ubuntu" in the bios boot menu? (UEFI)