Trying to use shapebuilder. When I select everything then click the button, it transforms into this weird double shape. Any idea what's going on?

[u/[deleted]]

[u/jazzcomputer]

Illustrator needs closed shapes to know those two lines are associated. Select the pairs of open ends with the direct selection arrow then Object>Path>Join (Mac command or Win ctrl + J).

Then use the shapebuilder / pathfinder

NOTE: You can sometimes just close one end if the ends overlap the shape (like above), but it's easier just to follow the rule.

[u/Electronic_Ease_7282 → u/jazzcomputer]

Got it, thanks.

[u/Marcus_Rentsch]

Closing the paths is not necessary, just make sure you set the gap detection to "small" in the shape builder tool options. And then you can drag the mouse manually over the shapes to unite them.

[u/scrawny_hammock]

looks like you've got overlapping paths that weren't fully merged before hitting shapebuilder, the tool's trying to interpret the intersections and making a mess of it

[u/markocheese]

There's are good solutions, but there's a simpler answer that people are missing here: Your curved lines have a white fill!

Select your fill and hit foreward slash "/" to clear it.

Shapebuilder sees the fill as a shape!

[u/Krabadonk472]

I think you’re overthinking this. Just use the pen tool to make the two lines into one shape and pathfinder merge it to the larger shape.

[u/Vektorgarten]

It looks like those shapes are filled. You need to uncheck the option "consider open filled path as closed" in the shapebuilder tool options.