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.

Stone Bedrock

This comes in 13 variants: stone, stone brick, cobblestone, mossy cobblestone, dirt, coarse dirt, sand, sandstone, obsidian, brick, netherrack, nether brick, end stone.

Water Bedrock and Lava Bedrock

Those come in 4 variants that look the same but are solid in different ways:

"normal": Not solid in any way
"solid": Completely solid
"solid_top": The block as a thin solid layer at its top. Intended for ceilings
"solid_bottom": The block as a thin solid layer at its bottom. Intended for floors

All of them flow like source blocks, but they are "super source" blocks, creating normal source blocks.

Using them

Superflat

When creating a "Superflat" world, the IDs to use are:

"extrabedrock:bedstone:0" through "extrabedrock:bedstone:12", order as listed above
"extrabedrock:bedlava:0" through "extrabedrock:bedlava:3", order as listed above
"extrabedrock:bedwater:0" through "extrabedrock:bedwater:3", order as listed above

Note that the layer list shows them all as "air" because they don't have an item.

setblock

With the "/setblock" command, use the same IDs as for worldgen but replace the second ":" with a space; e.g.:

/setblock ~ ~ ~ extrabedrock:bedstone 8

Other Worldgen

When using a worldgen tool that takes blockstates:

extrabedrock:bedstone{variant=xxx} where 'xxx' is the ID of the vanilla block (e.g. "stone")
extrabedrock:bedlava{variant=xxx} where 'xxx' is the variant as listed above (e.g. "solid")
extrabedrock:bedwater{variant=xxx} where 'xxx' is the variant as listed above (e.g. "solid")

Depending on your tool the syntax may vary.