One of the biggest YouTube videos of the year, YouTube's own year-in-review video "Rewind," starts as a spreadsheet. It's 400 rows and 10 columns filled with the names of videos, YouTube creators, songs, memes and trends that sum up the year in pop culture, a long list of references compiled from a similarly long list of videos that YouTube passes to Portal A, the production company that's produced every Rewind video since 2011.

"The first thing we do as a team is we watch everything," said Kai Hasson, creative director at Portal A. "There's a period of two-and-a-half weeks or so that we're just watching a lot of videos."

There's good reason for the deep-dive. YouTube Rewind may have been conceived in 2010 as a way for the Google-owned video service to showcase the year's top videos on the service. But as YouTube and the videos people upload to it have penetrated deeper into the mainstream, Rewind has become as much of a showcase of YouTube as of pop culture.

"When I think about what Rewind this year represents to me, it's YouTube stars in particular, but YouTube culture had maybe its biggest crossover year we've had," said Kevin Allocca, YouTube's head of culture and trends. "And the line between mainstream culture and YouTube culture are almost nonexistent,"

As an example of that line-blurring, this year's Rewind video highlights a meme that's not readily associated with YouTube: The Dress. While it spawned a bunch of parody videos on YouTube, the black-and-blue or white-and-gold garment that broke the internet back in February originated on Tumblr and gained attention on BuzzFeed. But that didn't really matter.

"Rewind is a celebration of culture and the internet," Mr. Hasso said. "One of the amazing things about the project is that it's not simply [a matter of] 'was this a YouTube video?' The way we all view YouTube, it's big enough where it kind of houses everything."

So The Dress made the list of references that Portal A needed to include in Rewind. So did Sia's "Elastic Heart" music video, the karaoke competitions popularized by "The Tonight Show With Jimmy Fallon" and The Weeknd's hit song "Can't Feel My Face." The next challenge was figuring out how to mash these seemingly disparate references cohesively into one relatively short video.

"There's like a hundred creators and hundreds of memes in five minutes. So how do you check off all those boxes and still make an enjoyable video to watch and not feel chaotic?" said Portal A managing partner Zach Blume.

That's where the spreadsheet comes in.

After everyone at Portal A watches the videos, the team creates presentation documents and spreadsheets that "break those references down into props, wardrobe, sets, anything that sticks out," Mr. Hasson said. That process helps to filter out the most memorable aspect of any video "because you need to be able to communicate it very quickly." It also helps Portal A's team identify how to combine references into segments of the video.

When it came to combining the aforementioned references, Portal A decided to pit rival groups of YouTube stars dressed in blue-and-black or white-and-gold lip-sync battling in the cage from "Elastic Heart" while a remix of The Weeknd's song by The Hood Internet plays in the background. These meme mashups enable Portal A to weave in references that mainstream audiences should get with YouTube stars or other memes that only hardcore YouTube viewers may recognize.

"We kind of shape our video around core references. Like 'Elastic Heart' and [The Dress], that's a section of our video that most people will probably be able to get in some way," Mr. Hasson said.

"If my parents can at least watch it and understand what's going on, it's a win," Mr. Allocca said.

The opening to this year's Rewind offers another example of these meme mashups. The video begins with a skeleton dancing to a mix of OMI's "Cheerleader" and Justin Bieber's "What Do You Mean?", a nod to the Ad Council's award-winning "Love Has No Labels" campaign. Then YouTube star Lilly "Superwoman" Singh pops out from behind the screen and dances in front of a wall painted like the LGBT Pride flag before jumping into a ball pit a la fellow YouTube star Roman Atwood's prank videos.

In another segment designed to give the video more of a global feel, Portal A shot a number of Latin American YouTube stars dancing on a bus cruising the desert outside of Mexico City -- a wink to the music video for Major Lazer and DJ Snake's "Lean On" that plays in the background -- while YouTube-star-turned-TV-host Grace Helbig does some carpool karoake with "The Late Late Show" host James Corden in the front seat and fire erupts from an electric guitar atop the bus "Mad Max"-style.

And for a sequence called "The 100 Years of Nae Nae," Portal A combined the "100 Years of..." format that gained steam on YouTube this year with the Nae Nae dance that's hit the mainstream over the past year following rapper Silento's song "Watch Me." Portal A filmed a group of 10 dancers performing the same routine set in five different time periods: the 1920s, 1950s, 1960s, 1980s and present day. "At the same time we're in a basketball gym, and we have these banners hanging from the top," Mr. Hasson said. So Portal A inserted different references, or "Easter Eggs," within the banners.

"It's making every frame of the video meaningful and a reference, so you can go back a bunch of times and see something new every time," Mr. Hasson said.

"It's like a puzzle," Mr. Blume said.

Classic YouTubers
YouTube's 10th anniversary added more pieces to this year's puzzle. To incorporate the milestone, the video actually rewinds through the video service's history while an original mix from EDM DJ Avicii plays. "The crescendo of the video is all 10-year stuff," Mr. Blume said. "It's an homage to the last 10 years of YouTube, and we go year-by-year almost like a countdown of maybe the biggest video from that year with current YouTube stars or the original viral star recreating that moment," Mr. Blume said.

As part of the 10-year look-back montage, Portal A brought in some of the earliest viral video creators, like the guy behind the "Double Rainbow" video and Judson Laipply who made "Evolution of Dance." "When I think of what's the biggest YouTube video of all time, I think 'Evolution of Dance,'" Mr. Hasson said. So Portal A recreated the video's set with a segment starring Mr. Laipply and more recent YouTube stars Rhett and Link.

"The fun of the project is the challenge of getting these cool memes and videos and references in there and seeing if you can top the year before," said Mr. Hasson. "It's kind of a project that's tailor-made for who we are and also the project itself has sort of become who we are."

When Portal A produced its first Rewind video in 2011, the company was two years removed from its basement beginnings. "We didn't come from the agency world or the big production world. We kind of just started making videos that we put on YouTube and made us happy," Mr. Blume said.

Those videos eventually caught YouTube's attention when it was looking for a firm to produce Rewind, and now Portal A has used Rewind to capture other companies' attentions. "Usually in the past we've scheduled out the year and know that the last quarter was Rewind time. This year we had a ton of other stuff going on at the same time," Mr. Blume.

In addition to Rewind, Portal A has been working on an original show starring The Gregory Brothers, shot in eight different countries in Asia; a year-long branded-content project with Lenovo filmed across three continents; and a gaming competition show for Google's iTunes rival Google Play. But Portal A may not have gotten any of that work without Rewind.

"It's nice to have a flagship project. It's meaningful for our company," Mr. Blume said in a recent interview at Portal A's office in downtown Los Angeles. "It's helped to define our identify in some ways. Just this morning I pitched a gigantic brand, and we're the YouTube Rewind guys. It's a pretty great place to start any conversation."