You're about to get a complicated answer to a simple question, lol....
Early on, my audience was largely built from a mix of Tapas community people and existing fans of my work. I've been making comics for over a decade, and have a small, but loyal fanbase for my art, as well as some people who like my D&D content or know me as a VA from Let's Read Homestuck and so checked out the comic when I launched on Twitter, and benefitted from some signal boosts from old friends off the UK comics scene. From these sources and regularly promoting on social media, I was able to get over 250 subs in the first 9 months or so of updates...
And then I started getting featured by Tapas. So after that point, the bulk of my numbers come from Tapas itself, but the core readers who leave the most comments come from the methods mentioned earlier; social media promo and the Tapas community. Readers off the app more often just read quietly and only sometimes turn out to be people who leave likes and comments. I still also do things like banner swaps with other creators, which also brings in people already on the Tapas app or site, and if there's a good match in vibe, the results can be quite good!