Is decentralized Blockchains are immutable?

I've often heard about the supposed immutability of decentralized blockchains, but is it really true? Can blockchains really never be altered or changed? I'm curious about the underlying mechanisms that supposedly guarantee this immutability. After all, isn't technology always evolving and subject to potential vulnerabilities? Could hackers or malicious actors somehow compromise the integrity of a blockchain? And if so, what are the safeguards and measures in place to prevent such occurrences? I'd like to understand the nuances of blockchain immutability in a deeper and more technical sense. Could you elaborate on this concept and address my concerns?