This is achieved by removing the requirement of every node on the network to store every piece of data, as most blockchains do. Nodes on IPFS opt-in to only the specific data that they’re interested in. Arweave distributes data between a subset of all nodes, ensuring a level of redundancy. In both cases, files are accessed using the hash of the file, rather than using a location on a filesystem on a specific server. This means that changing the file also changes its address. You can trust that a file appearing at a particular hash will always be the same file. If the file changes, so does the hash, meaning the reference to the file must be changed. This provides assurance that the file you were served is the one you expected.