From GPT:

An upside-down cross in St. Peter’s Basilica is not a symbol of disrespect or Satanism (as it might be misused elsewhere). Here, it represents Saint Peter’s martyrdom.

According to Christian tradition, Saint Peter, one of Jesus’ apostles and the first Pope, was crucified upside down because he felt unworthy to die in the same way as Jesus. Because of that, the inverted cross became a symbol specifically linked to Saint Peter and humility — not evil.

So:

•Two upside-down crosses on the doors?

•They are probably marking the Confessio (where Saint Peter’s tomb is located directly beneath the high altar).

•It’s emphasizing that the Pope (or the Church) traces its authority directly back to Saint Peter.

Summary: The upside-down crosses are honoring Saint Peter’s sacrifice and symbolize humility and the apostolic foundation of the Church.