The preparations are complete and the Grail has been brought to life once again.
Archer is fed up with Shirou’s stubbornness and refusing to see what will happen in the long term. He’s been preaching to Shirou for the past several episodes, but now that he has found his way, his spirit is stronger than ever. Archer has already committed so much to his plan that he himself is too stubborn to back out as well. The two of them have the thickest of skulls and just keep clashing against each other, the other refusing to accept themselves.
Archer used to be the same way, refusing to lose faith in his ideal, and continued along his mission on being a hero of justice. Somewhere along the lines, his faith wavered as he saw nothing but blood shed being caused by his own hands. It’s been so long since seeing that level of determination from someone that it even begins to make him question himself. He was once in that position, but even he faltered, which makes him hesitant to accept this Shirou’s determination to never have any regrets.
Archer ultimately accepts his defeat. While they’re technically the same person, his spirit is not as strong as his past self’s. Somewhere along his journey, Archer forgot who he really was and stopped believing in his dream. Seeing himself in the past was like looking into an old mirror and unlocked his memories. It seemed like he hesitated for a bit after being reminded of what he used to believe in.
At the end of the battle, the ‘real’ Archer appears. Gilgamesh is tired of seeing Shirou and Archer use copied weapons and unleashes the real deal. Archer saves Shirou in the barrage before disappearing and entrusting everything to Shirou.
Gilgamesh reveals the true nature of the Grail that everyone had been fighting over. He was a remnant of the previous war, somehow remaining in the world after it had all come to an end. He informs them of the true nature of the Grail, and the whole purpose behind the war. The Grail everyone seeks is right in front of them the whole time, but what they need is to fill the Grail with mana, by sacrificing servants with the war.
Saber was led to believe that obtaining the Grail could help grant her wish, but in the end, she isn’t even capable of using it. It’s a mass cultivation of mana, capable of anything when in the hands of a magus. In the end, she is nothing but a tool for the mages to achieve this limitless supply of mana.
The Grail itself also brings upon destruction. It has already been tainted and brings nothing but evil and curses towards humanity. Kiritsugu realized this at the end of the last war, which was why he had Saber destroy the Grail in the first place. Gilgamesh wishes to cause the same disaster as ten years ago as a way to cull humanity. Only those who survive are worthy of serving him.
Shinji is chosen to be the vessel for the Grail by Gilgamesh. He managed to scrape by for so long despite having such a loud bark and no bite. Karma finally finds his way to him as Gilgamesh uses him as a sacrifice. He was so full of himself throughout the series that he failed to see that everyone was just toying with him. Kirei and Gilgamesh made him feel important by lending him a hand, but in the end he was never in control of anything.
You can watch Fate Stay Night UBW Episode 21 on Crunchyroll