As far I can tell Steam does have two mechanisms. First is chunk based matching using rolling checksums, which is something like zsync/precomputed rsync. This a

As far I can tell Steam does have two mechanisms. First is chunk based matching using rolling checksums, which is something like zsync/precomputed rsync. This allows to switch between arbitrary build manifests and resuses matching chunks from previous file. Second is based on computing patches with smaller granularity, but generation of these is only triggered when enough clients download the update. They are more efficient but valid only for updating between exact build manifest pair they were generated for. When compressed packfiles are used they should be built with equivalent of gzip --rsyncable option, which restarts compression stream based on rolling hash values. Otherwise patching methods are defeated. For example UnityFS packfiles with chunked LZ4 compression normally don't do that, which results in huge updates. However it is possible to repack them into patch friendly form while keeping file format compatibility.