At the end of every year, DJ Earworm mixes together the top 25 songs of that year into a mashup. It's a clever idea, and it has the potential to be challenging - rather than choosing what songs would fit well together based on harmonic structure, melodic pitches, rhythm/meter, etc., the songs are chosen for him by their place on the Billboard chart. To be clear, these are not what I call megamix mashups in the style of Girl Talk. A megamix mashup is constructed like a combination of a basic mashup and a medley: each basic mashup section transitions into another section, usually with some overlapping parts (see my MTO article for more details). The United State of Pop mashups, on the other hand, function more like basic mashups, even though they are constructed from a large number of songs. Earworm generally uses 2-4 songs as the starting point for his mashup, and sprinkles bits of the others in like samples. This year, the track is constructed mainly out of "Despacito" by Luis Fonsi and "Shape of You" by Ed Sheeran.
One glance at the chord structure of "Despacito" makes it clear why it serves as the harmonic basis for the track: "Despacito" is constructed entirely around the progression i VI III VII, one of the most commonly used chord progressions in popular music (see "Zombie" by the Cranberries or the chorus of "Listen to Your Heart" by Roxette). It's actually a rotation of the "Axis of Awesome" progression (the fact that it starts on the minor chord establishes "Despacito" as being in a minor key). "Shape of You" doesn't follow the exact same progression, but the synthesized mbira loop that continues for the entire track fits well with those chords. In fact, a quick YouTube search reveals that there are several "Despacito"/"Shape of You" mashups that predate this year's United State of Pop.
After the introduction (where "Body Like a Back Road" by Sam Hunt is featured), the other 23 songs are spliced in like samples, most noticeably in the vocal line. Even with all of these contributing tracks, the lyrics still make sense, and Earworm posts them in the video description.
I like this mashup, but I wonder how much of that is based on the fact that I think both of the main source songs are so good by themselves. Earworm's talent in this particular track is not in recontextualizing melodic/harmonic material, but in his virtuosic sampling skills. Putting together 25 songs is a difficult task, nomatter how you choose to do it.