How to install Ubuntu as a multiboot system?

I have Windows 10 (official and insider version) on my laptop twice as a multiboot system. Now I want to install Ubuntu (maybe more Linux distributions). How do I need to install it so that the Windows Boot Manager remains selected and not the Grub Bootloader is installed or used? And what do I have to select if the installation of Ubuntu comes with the Secure Boot setting?

So you can't install the Grub Bootloader at the end of the installation. And what do you mean by the Secure Installation, what does he ask you?

Install Ubuntu on your own hard drive and then install the grub on it. Then by default the Windows Bootloader is loaded. If you want to start Ubuntu, then you have to select the Ubuntu hard drive in the BIOS boot menu.

But have on both hard drives already something? Is that synonymous with an extra partition? If so, how should I proceed?

Why do you necessarily want the Windows Booter?

Take the grub there's (almost) everything automatically.

This is my hard drive and goes absolutely clean with Grub:

Because I find the Windows Boot Manager somehow nicer

Nice outside, but ugly inside.