I don’t know the full answer, but Pegasus isn’t one single piece of spyware, but rather a toolkit of many, many zero-day exploits.

A lot of them (the majority maybe?) are non-persistent meaning that they don’t survive a reboot.

That said, aside from keeping your phone up to date with security patches and rebooting frequently, I’m not sure there’s much the average person can do if you’re actively being targeted.