Literature search: Immutability

I am trying understand some technical aspect of ethereum (generally PoS maybe) and would appreciate any pointers to literature that I should look at. I will try to define terms in case my usage is non-standard (I am not a crypto expert; just someone with a degree in cs/math).

A blockchain is immutable if it is prohibitively expensive (by some metric) to change any part of the entire history of transactions. (Please enlighten me if I am using the word "immutability" improperly here).

How is immutability in this sense enforced on a PoS chain? I can make sense of how it can work in a PoW setting but not for PoS. Any articles where I should start? Preferably a source where this aspect is discussed specifically. (My google-fu was not good enough).

Thank you!