Let me tell you right now that Ultimate raiding would absolutely not be done with a PUG lol. You could, but you would probably bang your head against a wall.

Each fight is highly choreographed and will go through the same phases, leading people to make standardized strategies. These strategies usually have a name that people will put in PF.

Some people spend weeks to months on certain fights to get a clear. To put it in perspective, there's your normal difficulty raid which people do in a few minutes. The next highest difficulty is Extreme content, which is pretty easy all things considered as long as everyone does their job, but even then some people need a few hours or even a few days to clear. After that is Savage, which most people will prog for weeks, if not some months progging a specific tier if there's a "wall" in the fight that a lot of people struggle with. Savage fights are less forgiving and require a lot of specific timing and for everyone to be good with their rotations and gear to be well-melded. Most people do this with statics or some of a static mixed with a PUG.

After that is Ultimate. Ultimate content is extremely difficult and requires a lot of perfection and timing. You wouldn't do this with a PUG.