Building the Pantheon today would be a demanding feat, imagine building it 2000 years ago and even imagining the dome would hold up when you started the work, much less for 2000 years. Whoever the architect/engineer was, they were unbelievably bold.
Concrete is strong under compression, but has weak tensile strength. Rebar significantly increases the tensile strength of the structure.
It still has the world’s largest cupola, and we have still don’t know how to build one that big, otherwise someone would have already done it.
So, it would not be possible to build such a building today, even though they did build one thousands of years ago.