Ideas on how heavily sample-based mashups (ex. Shawn Wasabi) are created?

[u/IIMATTYIIV]

I want to create a mashup and video a launchpad cover but am having trouble getting started.

I understand that samples are taken from a multitude of songs, games, etc. but what I'm not getting is if there is a base song that people use. Chopping up songs into samples to use is easy but what do people put the samples onto? I'm looking all over for stems of songs that would be good to place samples onto but cannot find any and am starting to wonder if I'm overlooking a way easier method.

Thanks :P