I recommend reading this short essay to understand what self-propagating systems are:

https://theanarchistlibrary.org/library/ted-kaczynski-why-th...

Here is an expanded version of the same essay as well (part 5 is prescient):

https://www.thetedkarchive.com/library/ted-kaczynski-why-the...

Also, a very lucid synthesis of TK's work is provided in Chad A. Haag's book: The Philosophy of Ted Kaczynski.