Fly question

So I'm not a fly fisherman but sometimes I will put one under a bobber. Last weekend I was at lake Medina and at sunrise fish were jumping all over the bay at the park, but they wouldn't touch anything I tried including a large, gray fly (I think it's a mosquito?). I think they were bass or white bass, some of the splashes were pretty big and they looked too big for sunfish, but they wouldn't touch my lures either, though I caught some fish later on them.

Anyways, if bass are feeding exclusively on insects what kind of fly might be effective? I caught several sunfish last year on small, black flies that I assume look like gnats to them, would bass go for those too? With the bluegills it didn't even have to be floating, once it got wet I would just put it a foot under the bobber and they would hit it aggresively.

Sometimes a school will all be keying on a swarm of bait of some sort (insect, fish etc.) and all they'll hit is that one shape/color/size. It can be frustrating finding what they are looking for.

You need a keen eye to find what they are looking for. That's part of fly fishing.
Study the area closely, Ie baitfish, downed insects, terrestrials, aquatic bugs. It's spring and could be anything this time of year.
And yes bass do eat bugs, a lot of them.

Try this for starter suggestions
P.S. get a fly rod it's much more fun!

hatch

Maybe some type of caddis, or mayfly.

Fishing conventional tackle you can always go with a zara spook, or something that mimics a smaller fish going after whatever is on the surface.

Sometimes it's as simple as shaking a bush on the bank to see what flies off it. Done this many a time on the Arkansas River in Colorado to turn my fishing into catching.

Bass love dragonfly nymphs. They even love dragonflys that are hovering over the water.

I like throwing a fly called a rubber legged dragon. It's got bead chain eyes and like the name says, rubber legs. The bead chain eyes get the fly down in the water column.

Another fly to consider would be a gummy minnow. I cast it under an indicator and slow strip it.

If they're chasing schooling bait fish, you have to hit the exact spot where they hit the surface, exactly when they're there, with the exact size and shape of what they're hitting, or you won't get nothing. But if you do get something, it will be instant.

I've often tried flies under popping corks or with a drop shot, and it just never works as well as a fly rod. I think the motion and the presentation isn't just right.

Your bobber needs to be at least 6 ft from the fly, lot's of leader. Use 5x or 6x tippet and even fluorcarbon.

As a strictly fly guy, I second drift4. Dry fly or flies on the surface require a leader 7 to 12 ft long. An intermediate line that sinks very slowly I usually use a 5-6 ft leader. If I'm down 3 feet deep or more I will use as short as 3 ft. For the surface action, a fly line is much less scary than a bobber.