The Egyptian forward struck twice inside three second-half minutes in Seoul, both created by Antoine Semenyo, as City overturned a first-half deficit against Diego Simeone's side.
Marmoush explained how a change in his own positioning after a quiet first half helped unlock the breakthrough.
"I had three, four chances when Savinho and Semenyo dribbled and they played the ball, and I wasn't there," Marmoush said. "So in the second half, all that was in my head was just to make the runs and be there, and they did their job, and we won, it was a really, really good feeling.
"I'm enjoying myself a lot. Obviously, as a striker, it gives you confidence, and it gets you going in the flow, and I just had to tap it on the net, and I'd like to thank Antoine, of course, and yeah, it just gives me confidence in the preseason. It feels good, next is the Community Shield."
Marmoush also touched on City's ongoing adaptation to life under new manager Enzo Maresca, who replaced Pep Guardiola over the summer after a decade in charge at the Etihad Stadium.
"We're trying to learn the new tactics, the patterns, and what the coach demands from us, and I think we're starting to get the hang of it," he said. "Obviously, after 10 years, what Pep did here was unbelievable. It's not easy after the change, but we've done really well, and we'll go back home and keep going."
The 26-year-old also reflected on a summer that included his wedding, as well as Manchester City's warm reception on their tour of Hong Kong and South Korea.
"The summer was amazing. I also got married as well, so I had the best days of my life and it's amazing to be back," Marmoush said. "The culture here is so nice. I'd like to thank all the fans for this unbelievable support.
"I felt the warmth. All of us felt it. The stadium was full today. Their culture is amazing. The food. I love their food, and we had a couple of amazing weeks."
City's Asia tour also included a draw with Inter Milan and a win over the K-League All-Stars, with Maresca's side now returning to Manchester ahead of the Community Shield against Arsenal at the Principality Stadium in Cardiff.