How secure is Bitcoin? How strong is the security of Blockchain? How immutable can it be?
Bitcoin survived not because it had not been hacked. It is being hacked 24/7 since day 1, and it survives. This is not saying that it won't be hacked in the future, such as the quantum computing possibilities certain parties are working with. The point is, if a system with its source code widely available to the public can survive for the past 9 years, it is needless to say how strong it is, by design.
Use the centralized banking system software system to illustrate the scenario. Imagine a big bank's computing system source code is made public. That is a major disaster. Theoretically, when people gain access to any system's source code, the system is doomed. That was before the born of Bitcoin, or the first Blockchain.
If you would like to know further about Blockchain and its security-related explanation via video, you can check out Manuel Stagars work here: http://bcinsider.my/blockchain-and-us/
A brief illustration using some very simple graphics can be found here: https://www.slideshare.net/howtze/blockchain-insider-preview
If you would like to read through a more detailed explanation, check out this free sample chapter from the book Blockchain Insider:
https://www.slideshare.net/howtze/blockchain-insider-sample-chapter
p/s: You may want to check out Bitcoin's coding related info at https://github.com/bitcoin, or related documents at https://bitcoin.org/en/developer-documentation