They have various ways of aiming. None of which are as fancy as some of the utter shite people claim works for them in the pool world. All snooker players will say they find the line of the shot. Is that an aiming system? No, but its because they have been playing for so long that they are able to find the line of the shot instantly no matter how the balls are situated. They step into the shot with their back leg, not lead leg, and this is always in the same place for every shot in relation to the line of the shot. The left, lead leg for a righty, is then placed in a position that allows the hips and shoulders to be in the same place in relation to the shot line. Some like the hips to be square to the shot, others prefer a more closed stance and the leg goes further out in front. The foot of the lead leg has nothing to do with how they aim, so it varies where its pointing. Sometimes with the size of the table the left leg has to be positioned elsewhere, but they could easily play standing on their right leg only, showing the left leg is purely for stability purposes and has no influence in how they aim.
Now, for a beginner, they will be taught ghost ball, fractional, back of ball, and contact point-opposite side contact point aiming methods. But most will use ghost ball. Anyone can pick out the centre of the ghost ball, so they trust setting up the same way will align them to the correct shot line each and every time. This means, find the ghost ball, step in, for example with the right leg and foot pointing directly at this ghost ball, then set the left leg in a way that positions the hips how you prefer to the ghost ball and drop down. Any minor adjustments are made without thinking and relies on previous experience to be able to make them. You ought to try it. Their straight strokes just allow them to send the white where they intend more accurately. Its amazing how accurate you can be when you clear your mind and just rely on previous experience