They just... turn up whenever.
Sometimes, there's an intention behind it all - like, I pitched a 14-page oneshot to the Beyond 2 anthology a few weeks back, and when coming up with that one, I sat down with the list of rules (has to contain a LGBTQ+ character/theme, has to be either post apocalyptic or urban fantasy, certain scenarios are not allowed, etc.) and tried to come up with something that a.) followed the rules, and b.) was something I'd enjoy drawing. I picked themes I like (in this case, people who feel anxious/uncertain, unlikely friendships, cute use of magic, domesticity, etc.), and built the story around that.
Sometimes, it literally just turns up. I'm one of those people who can't really draw random things - if I put pen to paper and draw anything more complicated than hands (referencing my own for practise), that thing WILL turn into a story. If I draw a person, they get a story. My brain just picks them up and runs with it. I can come up with an entire personality, backstory and plot in between sketching in the right eyebrow and the left.
Grassblades started as "hm, I really like wandering samurai stories, I should do one of those some day" and then I ended up with this unwieldy monstrosity of a story that is going to take me years to finish.
I had a 24 hour comic turn into a 168-page graphic novel once.
It's like my brain is constantly picking things up and shaking them until ideas fall out. Sometimes, it's other stories that spark ideas for my own - sometimes it's music. Sometimes, it's a walk in the woods. Sometimes they just materialise out of thin air.
And I tell stories constantly. Like, some people sing in the shower. Me? I tell stories.