Is the world an illusion?
No. Calling the world an illusion assumes a truer world behind it that the appearance gets wrong, and there is no world behind the shared one. Objective reality is the single consistent state that all observers' agreed records pick out, and a further fact hiding behind it would be a difference no record could ever show. Ask whether the world in front of you is the authentic article or a copy, and the question dies on the way out, because nothing is available for it to be a copy of. Solidity is real as well, with an account of its own. Your hand stops at a table because one pattern declines to occupy a state another pattern is holding. Your conviction that you touched something is itself a record, written in you.
Also in Chapter 2: Why Is There Anything Rather Than Nothing?, Chapter 35: What Is Anything Made Of?.
This answer comes from OPH, Observer Patch Holography. OPH is a rigorous, consistent and complete description of the unified, self-creating computational Universe we inhabit. Its consistency is machine-checked: the fixed point at its core is constructed explicitly, and a theory with a model contains no contradiction.
To understand OPH and check every step yourself, start with the free book Reverse Engineering Reality. It builds the theory from the ground up, and the OPH Notebook answers your questions while you read.
read the free bookask the notebookor ask sage:read chapter 11: What Does Everybody Actually End Up Holding?watch a universe make its own particles