Very different is not uniquely different and unrelated.
A different Kingdom, but like Plants and Animals, they share the same “Universal Code” that translates DNA genes into functional molecular machine proteins, and have the same ubiquitous “housekeeping” genes.
Now the genes that they share in common have more (mostly) superficial differences that accumulate, but they share genes in common with all other living things on Earth. The most parsimonious explanation for this is they once shared common ancestry.
The “original cell” idea is pretty much discarded. LUCA - the last universal common ancestor is no longer thought of as a particular species or type of cell, but a community of slightly different cellular organisms that swapped genes with each other.
So, allow me to expand on ‘very different.’
Archaea are at least as biologically different from bacteria as you are from a coconut tree.
Have a nice day.