Many different explanations have been suggested about what Jesus meant by this prediction. Most scholars and commentators agree He was referring to the "transfiguration," literally the very next event mentioned by Matthew. Jesus will be transfigured from His fully human form into His glorious kingdom appearance as the Son of God. His face will shine like the sun, and His clothes will become white as light (Matthew 17:1–2). Peter, James, and John will be eyewitnesses to this event.

A smaller number of interpreters believe Jesus was implying that the disciples would not die before seeing Him resurrected, or seeing the coming of the Holy Spirit at Pentecost, or seeing increasing numbers of people believe in Jesus following His resurrection and return to heaven.

An even smaller number of scholars connect this prediction of Jesus to the destruction of the Jewish temple in AD 70 or to the second coming of Christ.

The transfiguration seems to be the most natural fit for Jesus' prediction here, especially since it follows so quickly after this moment. Peter, James, and John are truly given a glimpse of Jesus in the full glory of His natural state in the kingdom of heaven—something extraordinarily rare among living people.

https://www.bibleref.com/Matthew/16/Matthew-16-28.html

Hmmm, this one is deep. Maybe we should read Jude 9, Matthew 9:20-22 and Acts 5:15-16.
My view is that there is a relationship between the person who has become close to God and the physical space of that person?
Is it some sort of placebo effect? Is there something with a deep spiritual meaning when someone believes in something very deeply? Is it faith?...
I’m a learner... I’d like to know.