A block is connected or chained to the previous block by using the previous block's hash and connected to the next block by providing it's hash. So the immutability comes after the fact ie once the block is created.

So at N + M block, changing the Nth block means all M blocks will become corrupted and needs to be changed or recalculated. The consensus or the p2p will come in the way of re-doing the blocks as everyone except the attacker will be at N+M. So unless everyone agrees there will be no way to change those blocks ie forced immutability.

Though the part people have hard time getting their head that there is never a single blockchain in existence. At a given point there are multiple blockchains existing but only the block with "most work" or the most difficult to calculate hash is added to the main chain. That is why the "confirmations" are required for a payment. The older a block gets, the probability of it getting added to the chain is higher. In bitcoin depending on the site, some take 6 confirmations or 6 blocks before being sure that the coin cannot be double spent.