I’m not sure why you want to use rEFInd, but that should not be a problem. Just install opensuse normally.
During the Ubuntu installation there was an option select where the boot loader would be installed and I put it on the same partition as Ubuntu. rEFInd then detect and boots the kernel.
It sounds as if you did a legacy install of ubuntu, instead of a UEFI install. It just happens that most of the important boot files go in the same place anyway.
I couldn’t find a similar option in the OpenSUSE installation.
It’s there if you do a legacy install, but not if you do a UEFI install. So presumably you did a UEFI install, which puts things in the right place anyway.e
How do I choose where to install the OpenSUSE boot loader? I won’t want it to install to the default location and possibly interfere with my working rEFInd set up.
It should not interfere anyway. So just install. Since you are using rEFInd, you are probably not using secure-boot. So disable the setting for secure-boot on the boot loader install options, though it won’t really matter either way.
Most of what is needed for booting is in “/boot” and “/boot/grub2”. Ubuntu uses “/boot” and “/boot/grub”. I doubt that it makes a difference, but you can create a symbolic link so that “/boot/grub” points to “/boot/grub2”.
The extra places that opensuse uses will be in the directory “/boot/efi/EFI/opensuse”. That should not interfere with rEFInd. If you don’t want it, you can delete that directory later. However a software update might later create it again. The opensuse install also adds an entry to the firmware NVRAM. You can delete that with “efibootmgr” if you don’t want it, though it might be added back with an update. Really, with UEFI booting, you can have multiple boot entries that don’t interfere with one another (unless the firmware implementation of UEFI is broken).
In your BIOS boot selection. If you legacy-boot the installer, it will use legacy booting. If you UEFI-boot the installer, it will attempt to setup UEFI booting.
Again, the differences are small. In the past, I have installed opensuse for legacy booting, but use UEFI booting anyway (with boot software from a different install). Most of the important files are in the same place either way.