Location unknown.
Muzan appeared with Giyuu in one of his import-export structures which he used to search for the blue spider lily and finance his research.
Mm, I'm going to put Giyuu in a bed in the mansion I own in the middle of the mountains, quite far from the city.
After installing it, he went to his office to plan his future plans.
While he was thinking about how to be able to walk in the sun. He will remember what happened in the final battle.
He saw him adopting this grotesque giant baby form, absorbing Tanjiro and Giyuu trying to prevent Tanjiro from being absorbed.
When he felt his end had come, and moved by Kagaya's words about the immortality of memory, he put his genes into Tanjiro and Giyuu.
Unlike Tanjiro, Muzan preferred to hide his genes deep inside Giyuu's body for more chance of survival.
As soon as he slipped his genes into Giyuu, he could see for a split second see Giyuu's memories. As soon as he saw him he shed tears, the times Giyuu spent with the pillars who for the most part despised him when he just felt inferior to them! He looks like me thought Muzan when he had been despised human because of his extremely fragile health. But as soon as Giyuu opened one eye and Muzan saw the color of his eyes he now wished to have him by his side.
This sky blue that he so desires was there in front of him!
As for Giyuu, he was just passed out and he had opened one eye from the pain he had felt when Muzan had thrust his finger inside him.
Eventually the giant baby Muzan disintegrated as soon as Tanjiro delivered the finishing blow to Muzan from within. Releasing Tanjiro and Giyuu in the process.
And this is where another fight began for the survivors.
To be continued
wiki/Muzan_Kibutsuji
wiki/Giyu_Tomioka