Resident Evil Nemesis EXPLAINED - Origins & Meaning
Defeat... But Not The End
Nemesis was defeated by Jill Valentine after it merged other infected material and became a behemoth of a monster. Taken down with a rail gun and a magnum shot to the head, this was just an example of what it would take to keep this creature down.
If the entirety of Raccoon City wasn't destroyed, Nemesis might have even recovered from this, though in some form, Nemesis lived on within Jill, as she ended up carrying a unique form of virus infection in her blood.
This was later utilised by Wesker to create Uroboros, so even after death, the legacy of Nemesis continues to terrorise the free world.
So that is everything about Nemesis and the Tyrants' origins leading into the RE3 Remake. We know Capcom plan on making some changes to the creature and its origins, but not what exactly.
Speculation is that they might try to characterise the monster more like the infamous movie adaptation, delving more into the human that existed before so much experimentation, but who knows.
For now, let's enjoy the fact that one of gaming's most fun and iconic villains is making a return VERY soon.