make a world like the end

[karmic kayak]

that's controlled in the dimension_type file, specifically by the effects field. you'd want to set it to minecraft:the_end

yes

data/<namespace>/dimension_type

no. as with everything else in a datapack you'll make a folder with that name and put your .json files inside that
seeing as you don't already have any other worldgen/dimension folders, i assume you meant you're trying to change the overworld? in which case you should put it in data/minecraft/dimension_type instead and name it overworld

the default for the overworld is #minecraft:infiniburn_overworld
if you are just replacing the overworld dimension type id recommend just getting that original file and only changing the effects field. you can get it from the vanilla jar or just in the presets in misodes generator