how do i make a dimension load a pre made map

i dont think so

Yes, you can.

make all the blocks in the dimension spawn as air and then when the player enters the dimension place the schematic of the map if the block at whatever relative position is air.

Simple, really.

no, the schematic can only be 32*32*32

I know that this is possible threw codding tho
Use this open SRC mod
https://github.com/Glitchfiend/Lockdown

I thought they removed that limit

no they didnt

You can make schematics larger than 32*32*32. I used my own method (inspired by the vanilla method 'nesting') to create a larger structure in Cogwheel Dungeons. Ask if you want to know how it works.

 if you want to know how it works. 

lol xd

What to I do once I have made the schematic so that it appears when I spawn in a custom dimension

1.16 its 48x48x48

 I have tested it and it works.

 BTW I think the only way is to include the save file in your mod download with instructions.

I know how to do this, idk if this would work tho, Basicaly, When the player joins the world, Make the player get teleported into a dimension which is just air, And then spawn a structure under them. Try making a procedure that replicates this process