What is a Mash-Up? Take one song, mix/produce it together with another song (or however many sources your want) and make a new composition. In some ways, it's like a cover and a remix thrown together.
Some are very good. Some are terrible. Some are done for humour.
Talk about the culture, talk about the producers, talk about the music... for me, Mash-Ups are pretty much the only way I know what Pop and Hip-Hop is current (I don't listen to radio or streaming services).
A lot of videos get nuked pretty quickly but an alternative source can usually be found.
I like Bootie Mashup as a quality control check for not terrible Mash-Ups. Their monthly and yearly compilations are usually well worth listening to... and since, like most DJs doing gigs, they've been affected by COVID time... so they are on Twitch now (it's a fun community with a huge crossover with the Critical Role fanbase).
Personally, I'm not very good at producing them... but I have made a few and re-created another because the download I had of it was of a shitty bitrate.