Underlay is a Minecraft mod that lets you place carpets (and more) under any block with space beneath it (e.g. chests, beds, signs, torches, etc.)!
Just right click a block face with a block to place it, (for example, under an upside-down stair), and you're done! Right click again with another item to replace it quickly, or left click it to remove it. Works in Survival too!
By default, you can place the following blocks as overlays: carpets, trapdoors, buttons, rails, slabs and pressure plates. To expand this to other blocks, such as grass, see "For Developers - Through Datapacks" (for players).
Underlay adds these things called "overlays" but they're really more like underlays xd, essentially they are fake versions of the item you place - which lets you place two things in the same block space!
In your Minecraft folder you can find config/Underlay/underlay.json. Use overlay_blocks to add blocks, and exclude_blocks to exclude them, like this:
{
"overlay_blocks": [
"minecraft:short_grass",
"mymod:shiny_block"
],
"exclude_blocks": [
"minecraft:oak_button",
"minecraft:rail"
]
}