Immersive Furniture: Custom Furniture and the Online Library
Immersive Furniture takes a different approach to decoration: instead of a fixed set of blocks, it lets you create custom furniture in-game or download from a vast online library. This guide covers how it works.
Crafting Material and the workstation
Everything is built from 
Crafting Material, made from an 
Iron Ingot, two 
Cobblestone and two 
Dark Oak Log (yielding a batch of sixteen). At the mod's workstation you use crafting material to build the furniture you design or download, so a steady supply of material is all you need to furnish endlessly.
The in-game editor
The standout feature is the in-game editor: rather than choosing from preset pieces, you create custom furniture, decorations and objects yourself, shaping exactly the piece your build needs when no preset fits.
The online library
Immersive Furniture connects to an online library of over 3,500 community-made pieces, with more added regularly. You can download furniture others have made and share your own creations, so your palette is effectively unlimited. Getting started means gathering crafting material, then either designing pieces in the editor or browsing the library for ready-made furniture to place.
The whole mod is two recipes
Immersive Furniture is two pages, two items and no blocks at all, built on two crafting recipes. That is not a summary — it is the complete content list.
The reason a furniture mod has no blocks is that its furniture is not made of them. What you craft are the tools, and the furniture itself is placed through the mod's own system rather than as individual block items in your inventory.
What that means practically
Searching a recipe viewer for a specific chair, table or lamp will find nothing, because no such recipe exists. Craft the two items the mod does add, and the catalogue is reached through them.
This is the same pattern several furniture mods use, and it is the single most common reason players think the mod has failed to install.
Both versions are the same
The 1.21.1 and 1.20.1 builds are both 0.3.3 and carry the same two recipes, so there is no version-specific behaviour to account for.
Where to start
Craft both items before deciding anything about layout. Since the mod adds no blocks of its own to place from an inventory, the furniture you can build is discovered through its interface rather than through a recipe book.