What an incredible place to visit! The enormity of the building, once inside, is even more amazing than the outside. Try to revisit in the evening too when the outside is lit up. Just wonderful!
We booked a guide for this and well worth it! Our tour guide Giorgio was wonderful! Hearing the passion and knowledge (from an ex history teacher and archaeologist) made it a great experience.
I know others might not be so lucky but a guide should be able to explain more about what you’re seeing.