I read this about 15 years ago.

I recall part of it set on a kind of prison planet where prisoners are just dumped and abandoned. The protagonist /possibly/ was an investigator or plant or framed rather than a true prisoner.

If you know any books that feature prison planets, please list them?

It's not *Chthon*, *Borders of Infinity*, *Stainless Steel Rat is Born*, *Prison Planet* by Dietz, *Last Starship from Earth*, *The Moon is a Harsh Mistress*, *Fool's Run*.

The next bits I describe may be from different books that I'm confusing, so don't base your suggestions only on fulfilling all these memories.

I recall a scene on a different planet set in something like an amphitheatre, possibly gladiatorial, where protagonist is in the audience.

I recall a possible later (final) part of the book where the protagonist interacts with a giant floating computer or space ship or space station.