The iso bios of Leap, TW, etc, are hybrid isos, that is, they can be booted in legacy rom mode or in uefi mode.
The motherboard has an option in the bio, which is called compatibility mode, (CSM) if the same usb pendrive is active, it appears as two different devices (being the same pendrive).
In the bios you can choose to disable CSM or choose Legacy rom (MBR) or a UEFI (Gui GPT) , or use the UEFI boot menu, which is a function key F that you press on post power on the pc, all devices appear there (depends on CSM).
With HDs, it would be the same if you keep the old fat partitions. almost better if you have nothing, use a partition program, or the one from yast.
The fat partitions in UEFI with their boot managers have a link to /bott/efi And there the possible directories and boot devices are housed Example one for openSUSE , another for Microsoft, etc… (in my case I prefer to have them separated and in the case of fat (which can be in 12, 16 and 32), there is no problem, openSUSE uses fat16 and win fat32, so they can be independent on different partitions.
It’s easy to see the devices (boot candidates, from linux) using the efibootmgr command:
HP-OMEN:~ # efibootmgr
Timeout: 0 seconds
Boot Order: 0001,0000,0002,9999
Boot0000* Windows Boot Manager
Boot0002* Solid State Disk
Boot9999* USB Drive (UEFI)
In this case, the order is 1st 1 openSUSE and the second is win 3th number zero.
I recommend UEFI, you have more than 200 or so primary partitions, gpt partitions with redundancy in the tables, and a long etc…
Secure boot if you don’t use it, you can disable it from installation or from yast boot and boot order the same, you can set it from yast boot.
This is a very broad topic, in my case I use a translator from Spanish to English, if I make a mistake, I apologize, also here you have many well-informed people who can help you with this.
Thank you and a very cordial greeting.