Extra Bedrock adds 3 new unbreakable, untargetable, unobtainable blocks to the game. It is targeted at modpack and map makers that need boundaries for their maps that are not bedrock.
This comes in 13 variants: stone, stone brick, cobblestone, mossy cobblestone, dirt, coarse dirt, sand, sandstone, obsidian, brick, netherrack, nether brick, end stone.
Those come in 4 variants that look the same but are solid in different ways:
All of them flow like source blocks, but they are "super source" blocks, creating normal source blocks.
When creating a "Superflat" world, the IDs to use are:
Note that the layer list shows them all as "air" because they don't have an item.
With the "/setblock" command, use the same IDs as for worldgen but replace the second ":" with a space; e.g.:
/setblock ~ ~ ~ extrabedrock:bedstone 8
When using a worldgen tool that takes blockstates:
Depending on your tool the syntax may vary.