Admittedly I've never given much thought to what the AI are doing in practice, other than when they get in my way of course 😂.
Based on what I do know about the AI, I would guess that it's not very sophisticated. All the AI cars use the same setup in terms of wings and suspension so I would assume they're all running the same fuel load. I would also assume there isn't any sort or pre-determined run plan as they don't have to complete practice programs to get R&D points, they just get points based on their driver's experience rating.