That doesn’t make sense. For a star to be that size, why would it go into red giant stage?

Stars of that magnitude go supernova far before they should ever reach red giant stage, like our sun. Our sun will swell large enough to engulf the earth, and possibly mars, then will shrivel down to the luminosity of the moon currently, then will dwindle into nothing more than a burnt out light source called a black dwarf.

But supermassive stars have a different timeline, they implode over themselves, and essplode, possibly making black holes, and what not.