Are you they actually appearing before you or are you trying to visualize them?
It can be both depending on the presence of the entity and the senses of the person. Some see them as if visualizing and others can see them as if “physical”
I personally do not often see a clear image of the demon. The experience of summoning is much more tactile and emotional for me, rather than visual. In the beginning I barely sensed any signs of a presence, and I still got results, so these things do not actually matter all that much. They can be enjoyable and can aid your practice in ways that you will discover over time, but getting measurable results is still the most important part of magick.
I know this is something that many beginners spend a lot of time worrying about and trying to get right, but in my opinion doing so is counterproductive. If you are persistent, you will have “supernatural” experiences, without any effort required on your part. All you must do is allow them to happen when they will.
I’m still new to the whole thing but I’m pretty sure it depends on what methods you are using, like if you’re using incense smoke to call them to physical existence or if you’re using a black mirror etc. etc. I do neither, so I’ll just feel a strong magnetic presence in the room and see/hear them with clairvoyance/clairaudience. Physically sometimes I’ll see the ground distorting or the room flashing different colors, but sometimes people don’t get such stimulation. I know that I’ve never had flashing lights or anything but some people get that.
I think it’s important to point out that there’s a difference between visualizing them and just receiving information. I’m not creating any visualizations. I think it’s not so different from perceiving other spirits