*exactly* how does flash work?

Even if you are not a coder, this series of 6 articles are a *fantastic* (probably the best I've read) on how flash drives actually work under the hood, and covers things like erase cycles, SLC, MLC, TLC (and how they differ physically).  Its also METICULOUSLY cited for even more depth.

Strongly recommend you consider reading all 6 parts.

http://codecapsule.com/2014/02/12/coding-for-ssds-part-2-architecture-of-an-ssd-and-benchmarking/