The XYZ buttons were small for a reason, the majority of SEGA games didn't even use them that much. the MD 6-button controller was basically designed just for arcade fightan ports, most games didn't use XYZ at all (let alone the MODE button lol).

ABC were the primary buttons, and it's what most games were designed around.
>In a typical platformer:
B would be cancel and run/shoot/grab/etc - C would be confirm and jump - and A would either mirror C, or it would be a tertiary button (like Select in many NES games). This is even how the Power Base Converter translated SMS controls to the 3-button pad.
There were some games that used B = Jump, A = Run/etc, and C = tertiary, but I think this was less common? Correct me if I'm wrong. At the very least, there WERE some games that had built-in control presets in the options, which usually let you choose between the two styles.

With the Saturn, most games still functioned like this (prioritizing B and C, with A as the "third" button), but it was still common for XYZ to be less-used inputs. Generally, the functions mapped to XYZ on Saturn would typically be mapped to L2/R2, Select, or the 4th remaining face button on PlayStation. Not much consistency in it, it depended on the game.

Ideally, if I were mapping a typical modern layout to a Saturn pad's XYZ ABC, it would be something like:
>L2, Circle, R2
>Triangle, Square, Cross