oneui-kernelsu.img: For OneUI ROM (Samsung MTP enabled)aosp-kernelsu.img: For non-OneUI (pure AOSP-based) ROM (Samsung MTP disabled)
Actually, recent GSI has MTP working fine even with Samsung MTP driver enabled. But just to be sure.
Flash it to boot partition with TWRP, then install KernelSU by backslashxx: https://github.com/backslashxx/KernelSU
How?
backslashxx described method to use KernelSU on older kernel version, such as 3.18. Here's the guide if you want to achieve the same thing: backslashxx/KernelSU#20.