Oh! Hum. Some of this answer is being a weird artist gremlin and having years of critiquing work. But that's not too helpful.
So, Like Betta Fish Do and a few others were chibis. I basically picked a scene from the chapter and drew a little snapshot of it. It was a fun way to do art for the fic but quickly.
Shadow of a Bat was mimicking the Animated Series. And the others... Well I try to either pick something symbolic from the title like in Cut Flowers and City Pigeons, or something interesting from the story. Tim Jr is sorta both. I also went with a specific retro style for that one. Offering on a Chance specifically I wanted something quick and accenting the red of lips and blood was really fitting. Masked is obviously very graphic and playing with the color change of the text for a reveal and hide sort of effect.
I'm not a huge text person, so you'll see the same font used a few times lol. I try to pick one with good vibes that has a little bit of interest itself. I like text that isn't white or black for interest.
Honestly, a lot of times it's just trying to do something quick and eye catching! It's a fun way to exercise that artistic limb too with different styles and things. Like Assembly Required is based on IKEA assembly manuals and keeping things simple. It was fun, even if it's not a style I'll go back to!