efibootmgr

Depends on what kind of user it is. My BIOS can select the disk that will be first tried for boot, and within that which partition has priority. I can select this during boot without setting it in the bios too. Another option is to use one of the boot disk on usb drive solutions. There are even alternative boot loaders (not windows or grub) that can search the disk for ALL bootable partitions.

A non-technical user does not mess around with partitions :)

Windows bootloader is at the mbr of the first disk. You have opensuse install its bootloader into its own disk. I am not sure if UEFI changes things here but if anything it probably makes things easier.

Both NTLDR and Windows Boot Manager are capable of redirecting to subsequent bootloaders, this feature is intended for dual booting with older versions of Windows but can also be used to dual boot into Linux and other operating systems.

As an example, if you install Windows 98, then Windows XP, then Windows 10 side by side, the following things happen:

1. Windows 98 installs its bootloader.

2. Windows XP takes a dump of 98's bootloader into a file and then overwrites the MBR with NTLDR. NTLDR is then configured with an option to chainload into 98's bootloader by reading its dump file.

3. Windows 10 takes a dump of NTLDR into a file and then overwrites the MBR with Windows Boot Manager. Windows Boot Manager is then configured with an option to chainload into NTLDR by reading its dump file.

This means you can install Windows 10 (and Windows Boot Manager), install Linux and have it install its bootloader (eg: GRUB) somewhere, then configure Windows Boot Manager to chainload the Linux bootloader (eg: GRUB) by reading from wherever it's stored.