What do we mean when we say that blockchain has transaction finality/immutability?
That it is consider extremely hard (impossible if you don’t count in a 51% attack) to reverse or change a transaction on the blockchain. That is if we consider the blockchain as in Bitcoins case to be extremely solid with a lot of mining happening to secure the network. If the network is safe and healthy then a transaction can’t be changed, reversed or deleted in any way shape or form, it is written on the blockchain to stay indefinitely as long as the blockchain is existing at a minimum of one computer somewhere in the future world. Compare it to be as close to or perhaps more future proof then how we perceive the words “written in stone”.
How does this lead to the trustless environment that blockchain creates?
Instead of me needing to trust a business or a specific person I can with blockchain put my trust in the network that run on the Bitcoin protocol, I still need to trust something but that something is more verifiable, open and transparent in the Bitcoin protocol. If I believe that the network and protocol is healthy I can then transact with whoever I want that is also using the Bitcoin network and only ever needing to trust the network itself.
Me still needing to trust the Bitcoin network is not the same as the trust I put into every other business I do today, to build up my trust in the Bitcoin network I can verify everything along the way to make my own prediction with the open information I can get today from the Bitcoin blockchain. Problems arise in todays society when that trust and future prediction needs to be done on a “black box” in a company where you cant verify anything by yourself to build up the trust in a healthy manner.
The trustless environment comes from that openness where you can verify everything and that the healthy network then also can guarantee the finality/immutability of the thing that you then verifies.