Does the OXS Thunder Pro have real rear speakers?

No. It is a single soundbar with eight drivers. The 5.1.2 effect is created from the bar itself, so behind-you and overhead sounds are simulated rather than coming from separate speakers.

What do the game modes change?

There are three tuned game modes and one standard mode. Each adjusts the tuning to make positional cues like footsteps and gunfire easier to place. The standard mode is meant for movies and music.

Is it good for movies as well as games?

Yes. It decodes Dolby Atmos and has a standard mode for films and music, so it works as a general TV soundbar. Its tuning leans toward gaming, though.

Do I need a receiver or extra speakers?

No. The soundbar handles Dolby Atmos decoding itself. You connect your TV or console to the bar and it produces the 5.1.2 effect on its own.

How does it compare with a full surround system?

A full 5.1.2 system with physical rear and height speakers places sounds more precisely. The Thunder Pro trades that for a simpler single-bar setup and less clutter.