I generally use a Magic Mouse. For years, the solution to prevent inadvertent trackpad touches — which then repositions your cursor and disrupts your typing/working flow — has been in System Preferences>Accessibility>Pointer Control and then checking “Ignore built-in trackpad when mouse or wireless trackpad is present.” Worked beautifully.
At some point (a couple OS versions ago, I think), it quit working very reliably. Wanting to investigate this again since I just picked up a new 16 inch MacBook Pro (with bigger wrist rest area and MUCH bigger trackpad), I have discovered that if I have freshly enabled the setting and then reboot, the setting seems to work unless/until I de-check it (at least for the last 5-10 minutes or so as I type this). Then, after being disabled (to actually use the trackpad temporarily for whatever reason) re-enabling it does nothing until I reboot, whereupon it works again.
Any fix for this? I guess complaining about having to reboot before a session of writing/typing is a real first-world problem, but would be great if I could squash this bug, or find a workaround.