Underground is record labels owned by a couple of friends pressing 250 records globally and distributing them by any means possible.

Underground is people putting all their spare money into putting on parties/gigs in obscure venues for the 100 other people in their city that like their particular niche of music/art.

Underground is not sitting in your room recording lip-sync videos and then sending them out over a platform used by millions.

if you are an artist performing live shows and you are interested in building a cult of followers you can do so, and no one will stop you. If your performances and merchandise fees break through a threshold of profitability, you are designated a success. You can use digital tools like YouTube and Facebook to do this, but engagement is low. There are new companies and digital tools that are building engagement by facilitating addiction and game mechanics to make artists money. Record companies are bad, and they are not the way forward.

Interestingly it requires the artists to be digital/business savvy which may select for those artists over those with better music, but that's always been the case to an extent.