Structures are split into finish and structure. Finish are the blocks that supposedly are used last in the structure (like wither_skeleton_skulls are placed after the soul_sand).
//Just a block and nothing else
"minecraft:diamond_block"
//You can enter here the blocks like when you would use the /setblock command
//if you want more details
//for example:
"minecraft:chest[waterlogged=true]"
//If you want to include multiple options (for example facing) it can be done like so:
//"minecraft:oak_log[facing=<axis if build was built in original direction>|<axis if build was built in other direction>]": ["0,0,0"]
"minecraft:oak_log[facing=x|z]"
//If you want to allow either x or z axis for one block, it can be done like so:
"minecraft:oak_log[facing=y+x|y+z]"